Drop-In Filter Mount Adapter EF-EOS R with C-PL Polfilter

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As a long-time user of the Canon EF sys­tem, I have mean­while a larg­er col­lec­tion of high-qual­i­ty EF lens­es. To be able to use them on the new Canon EOS R5 with RF bay­o­net, Canon has released dif­fer­ent adapters. Right from the begin­ning, I have been using the basic mount adapter on my EOS R5, but I was curi­ous about the pos­si­bil­i­ties of an adapter with extend­ed capa­bil­i­ties. Since I use a polar­iza­tion fil­ter very often, I there­fore ordered the Drop-In Fil­ter Mount Adapter EF-EOS R* with the C-PL polar­iza­tion fil­ter about 5 weeks ago.

After a long wait due to the tense deliv­ery sit­u­a­tion, I final­ly received and test­ed it. Here is a short report on the results.

In con­trast to the basic mod­el of the mount adapter, the Drop-In Fil­ter Mount Adapter EF-EOS R has a fil­ter slot, like those of the Canon Supertele lenses.

If some­body should think now: Great, then I can use my polar­iz­ing fil­ter of my Supertele inside, I have to dis­ap­point: I already use a polar­iz­ing fil­ter for my EF 400 f/2.8L IS II USM, but it does­n’t fit into the mount adapter despite the same inner fil­ter diam­e­ter - great job, Canon!

But before I go into more detail about the fil­ter adapter, a few thoughts about the pur­pose of pho­to­graph­ic fil­ters. If all this is already known, the fol­low­ing sec­tion can be skipped.

Why do you need filters at all?

With ana­log cam­eras (these are the ones that used to require the inser­tion of film 😉 ), the post-pro­cess­ing pos­si­bil­i­ties of the images were very lim­it­ed, so that fil­ters were reg­u­lar­ly used to com­pen­sate for col­or dif­fer­ences in arti­fi­cial light or strong con­trasts in bright­ness between sky and earth in land­scape shots (with grad­u­at­ed fil­ters). Espe­cial­ly when pho­tograph­ing with slide film, the pos­si­bil­i­ty of post-pro­cess­ing was almost com­plete­ly eliminated.

In dig­i­tal pho­tog­ra­phy, how­ev­er, espe­cial­ly with RAW files, col­or cor­rec­tion can also be per­formed eas­i­ly in post-pro­cess­ing. In the case of high bright­ness con­trasts, I often take an brack­et­ing series of 3 or more shots with dif­fer­ent expo­sure times and com­bine them on the com­put­er to cre­ate a shot that repro­duces the desired dynam­ic range well. To a less­er extent, this can also be done direct­ly from a RAW file, since today’s cam­era sen­sors have a much greater dynam­ic range than ana­log pho­to­graph­ic film. The exten­sive pos­si­bil­i­ties of dig­i­tal post-pro­cess­ing with RAW con­vert­ers thus make many fil­ters dis­pens­able - but not all.

In short: Since I am pho­tograph­ing dig­i­tal­ly, I use only two dif­fer­ent types of fil­ters on my lenses:

Grey filter (ND)

A gray fil­ter is what its name says, gray. In fact, some­times it is almost black. Its pur­pose is to reduce the amount of light reach­ing the sen­sor in a uni­form and col­or-neu­tral way (ND stands for Neu­tral Den­si­ty). In con­trast to the stop­ping down of the lens, which changes the depth of field, it does not influ­ence this. An image of non-mov­ing objects tak­en with a cor­re­spond­ing­ly longer expo­sure time using a gray fil­ter can­not be dis­tin­guished from one with­out it.

The pur­pose of the gray fil­ter is usu­al­ly to extend the expo­sure time. When film­ing, it is indis­pens­able to achieve a suit­able expo­sure time, which should usu­al­ly be half the dura­tion of the sin­gle frame (e.g. 1/60s at 30 fps), even in day­light and with an open aper­ture. Pro­fes­sion­al film cam­eras there­fore often have inte­grat­ed switch­able ND fil­ters of dif­fer­ent strengths.

When tak­ing pho­tographs, how­ev­er, a gray fil­ter is often used to achieve motion blur in day­light. This makes it pos­si­ble to dis­play water sur­faces smooth­ly regard­less of the waves. For this pur­pose, I use an ND 3 fil­ter, which only lets 1/1000 of the light through. Instead of 1/30 sec­ond expo­sure time you have to expose the fil­ter for 30 sec­onds to get the same amount of light to the sen­sor. In the fol­low­ing I want to show two exam­ples to illus­trate the purpose:

16mm, f/8, 6s, ISO 100 with ND-3 Filter

In the pic­ture above, the long expo­sure time of 6 sec­onds makes the indi­vid­ual water drops appear as light tracks, which makes the pic­ture look much calmer. Anoth­er exam­ple fol­lows here:

24mm. f/16, 13s, ISO 100 with ND-3 Filter

The image above was also tak­en with a 1000x grey fil­ter (ND-3) and an expo­sure time of 13 sec­onds. Due to the long expo­sure time, the sur­face of the water appears smooth, the wave motion is shown like a light fog­gy coat­ing. The clouds appear more blurred due to the move­ment, the image there­fore has a cer­tain calmness.

Polarizing Filters

A polar­iz­ing fil­ter allows light of only one oscil­la­tion plane to pass. Nor­mal day­light and also arti­fi­cial light con­sists of light rays that oscil­late in all planes. On non-metal­lic sur­faces, how­ev­er, the light that oscil­lates per­pen­dic­u­lar to them is reflect­ed more strong­ly. If the polar­iz­ing fil­ter is adjust­ed to a 90° angle, these light com­po­nents are blocked out. This effect can reduce reflec­tions on glass, water sur­faces and oth­er smooth surfaces.

This makes the green of the leaves look more intense and the sky turns to a dark­er blue col­or. This effect is most intense when the sun is at a 90° angle to the image detail, direct­ly against or with the sun, the polar­iz­ing fil­ter has hard­ly any effect. With strong wide-angle lens­es that cov­er a wide range of angles, the effect can there­fore vary in inten­si­ty in the dif­fer­ent sec­tions of the sky, which can some­times be disturbing.

There are still lin­ear and cir­cu­lar polar­iz­ing fil­ters to be dis­tin­guished, but to go fur­ther into this dif­fer­ence would lead too far. In the end, how­ev­er, in pho­tog­ra­phy at present cir­cu­lar polar­iz­ing fil­ters are used almost exclu­sive­ly. Fur­ther details can be found at inter­est e.g. at Wikipedia.

24mm, f/14, 1/80, ISO 100 with polar­iz­ing filter

The above image is tak­en from the same posi­tion as the one shown above with the ND-3 fil­ter. The post-pro­cess­ing in Light­room was the same for both images. But this time I used a polar­iz­ing fil­ter (B&W Käse­mann*). The expo­sure time was now only 1/80 sec­ond. In com­par­i­son, you can clear­ly see that the green of the leaves appears much more intense, the sky shows a slight­ly more intense blue, the clouds are more clear­ly struc­tured and the waves on the water are now per­fect­ly visible.

I often use the polar­iz­ing fil­ter for land­scape and city shots when the sky is clear.

Back to the Mount Adapter Drop-In

Why did I buy this adapter now? As a long-time Canon pho­tog­ra­ph­er I have a large col­lec­tion of Canon EF lens­es, which I would like to use on my new mir­ror­less Canon EOS R5 as well. In addi­tion I want to con­tin­ue using my proven Canon EOS 5DSR DSLR which can only be used with EF lens­es. Some of these EF lens­es are also not yet avail­able for the new RF bay­o­net. Canon was aware of this fact when they intro­duced the EOS-R sys­tem and there­fore offered the pos­si­bil­i­ty to con­tin­ue using the exist­ing EF lens­es via EF-RF adapters.

In con­trast to third-par­ty solu­tions like those from Sig­ma (MC-11) for Sony alpha cam­eras, the EF lens­es are even native­ly sup­port­ed on Canon R cam­eras. There­fore the per­for­mance remains at least on the same lev­el as with the Canon DSLR bod­ies. Some lens­es, like my TS-E 17mm or the 85mm f/1.2L II even work much bet­ter on the EOS R5 than before on my DSLR. More details can be found in my ini­tial expe­ri­ences report about the Canon EOS R5.

Comparison of the filter types

Zirku­lar­polfil­ter - links Canon Drop-In C-PL, rechts B&W Käse­mann 77mm

In the pic­ture above, I have placed the two fil­ter types next to each oth­er. The screw-on fil­ter on the right is a high qual­i­ty mul­ti-lay­er coat­ed mod­el from B&W Käse­mann* with 77mm thread. If you com­pare the grey val­ues in the pic­ture above, you can already see that the thread­ed fil­ter absorbs a bit more light than the drop-in fil­ter. Since both fil­ters are on a white back­ground and the incom­ing light there­fore pass­es the fil­ters twice, they appear dark­er here than in trans­mit­ted light.

Screw-on filters

Since many lens­es have dif­fer­ent fil­ter diam­e­ters, you need sev­er­al fil­ter sizes or you have to adapt larg­er fil­ters with adapter rings, which means a lot of fid­dling. Fur­ther­more, the lens hoods often do not fit anymore.

I have been using a very high qual­i­ty 77mm fil­ter (B&W Käse­mann) that fits on my Canon EF zoom lens­es from 16-200mm.

Even if the fil­ter diam­e­ters of the lens­es are iden­ti­cal, the fil­ters have to be changed as well or you need sev­er­al of them, which is quite expen­sive. There are also some lens­es which do not offer the pos­si­bil­i­ty to use screw-on fil­ters because of pro­trud­ing front lens­es or which have such large fil­ter diam­e­ters that fil­ters become dis­pro­por­tion­ate­ly large and expensive.

Cur­rent­ly I own 3 lens­es with this prob­lem: the TS-E 17mm, my Samyang 14mm and my Sig­ma DG OS HSM 60-600mm (with 105mm fil­ter diam­e­ter!). For the wide angle lens­es, you might use fil­ter hold­ers that are mount­ed in front of the lens, but they are also very bulky, expen­sive and dif­fi­cult to mount.

Using the filter mount adapter

The fil­ter in the fil­ter mount adapter fits for all EF lens­es and can remain on the cam­era when chang­ing lens­es. By the way, in addi­tion to the polar­iz­ing fil­ter I bought the Drop-in Clear Fil­ter A* which can remain in the fil­ter mount adapter if I don’t want to use the polar­iz­ing fil­ter. This has the wel­comed side effect of com­plete­ly cov­er­ing the shutter/sensor cham­ber when chang­ing a lens, so that dust can be effec­tive­ly blocked. I think this will prove its worth on dusty safaris (which we hope­ful­ly can plan again soon after Coro­na) in the future.

The ques­tion that inter­est­ed me now was whether the drop-in fil­ter is opti­cal­ly as good as my proven screw-on fil­ter, which alone costs more than $ 100,-. Does it per­haps make a dif­fer­ence, whether the fil­ter is locat­ed in front of or behind the lens?

Comparison test

In order to ensure iden­ti­cal con­di­tions of com­par­i­son, I inten­tion­al­ly chose a sta­t­ic test set­up with arti­fi­cial light. The motive, that was used, is a small object that is very famil­iar to every Canon user:

With­out polar­iz­ing filter

I put the lens cap on a white sheet of paper and illu­mi­nat­ed it from behind at an angle of about 45°. I aligned the cam­era on a stur­dy tri­pod at an angle of about 30° to the cap. All pic­tures were tak­en with the Canon EOS R5 and the EF 24-105 f/4L II IS USM lens at 105mm focal length, aper­ture 8, 1/8s expo­sure time and ISO 100 and edit­ed in Adobe Light­room Clas­sic 10 with iden­ti­cal para­me­ters. I adjust­ed the white bal­ance in Light­room with the pipette on the white back­ground of the image with­out mount­ed fil­ters to make any col­or devi­a­tions visible.

The fol­low­ing pic­tures show the ver­sion with the Canon Drop-In C-PL fil­ter on the left and the ver­sion with the B&W Käse­mann screw-on fil­ter on the right.

First a com­par­i­son with both fil­ters set to the min­i­mum effect:

Polar­iz­ing fil­ter in neu­tral position

First of all, you can see here, that the right image is slight­ly dark­er than the left one. There­fore I have used LR to bright­en the right image by 1/3 f-stop, in which case the bright­ness is comparable:

Polar­iz­ing fil­ter in neu­tral posi­tion, right +1/3 f-stop

All in all, the Canon CP-L shows slight­ly more details only in the 200% view. In direct com­par­i­son, the screw-on fil­ter pro­vides a slight­ly warmer image. But both effects are absolute­ly negligible.

And now the much more inter­est­ing com­par­i­son of both polar­iz­ing fil­ters using the max­i­mum amount of vis­i­ble effect:

Polar­iz­ing fil­ter at max­i­mum effect

Again, the image with the screw-on fil­ter is slight­ly dark­er, so here again the com­par­i­son with bright­en­ing of the right image by 1/3 f-stop:

Polar­iz­ing fil­ter with max­i­mum effect, right +1/3 f-stop

For­tu­nate­ly, in direct com­par­i­son both images show a com­pa­ra­bly good extinc­tion of the reflec­tions of the light source. This is sure­ly the main thing. Again, the screw-on fil­ter shows a slight­ly warmer image and shows slight­ly less detail, which is only vis­i­ble in the 200% view.

By the way, the qual­i­ty of the res­o­lu­tion can be objec­ti­fied indi­rect­ly via the size of the RAW files: Canon stores them inter­nal­ly in a loss­less com­pressed for­mat. More detailed images can be com­pressed less than images with less detail. Since all oth­er fac­tors (image con­tent, expo­sure para­me­ters, lens) are the same for all images, the file size is there­fore a suit­able para­me­ter for mea­sur­ing the lev­el of detail. The RAW files using the screw-on fil­ter are between 1.6% and 1.9% small­er than those using the slide-in fil­ter. How­ev­er, this min­i­mal dif­fer­ence is cer­tain­ly absolute­ly neg­li­gi­ble in real life.

Comparison under real conditions

In response to a spe­cial request, I com­pared both types of fil­ters under real con­di­tions. This time, dur­ing my trip to Africa, I took a pho­to of Mount Kil­i­man­jaro in Tanzania. 

The shots can be found below: on the left the shot with the Canon drop-in fil­ter and on the right the one with the B&W Käse­mann screw-in fil­ter. Both shots were tak­en short­ly one after the oth­er with my Canon EOS R5 and the EF 16-35 f/4L IS with a focal length of 16mm, aper­ture f/6.3. I adjust­ed both fil­ters to max­i­mum effect. The image with the screw-in fil­ter was bright­ened by +0.23 in Adobe Light­room for direct com­par­i­son, all oth­er slid­ers were left in neu­tral position. 

Inter­est­ing­ly, the screw-in fil­ter seems to give a slight­ly warmer tone. The WB-auto­mat­ic of the cam­era deter­mined a col­or tem­per­a­ture of 6250 °K there. With the drop-in fil­ter, how­ev­er, it was 6500 °K. But this min­i­mal devi­a­tion is sure­ly irrel­e­vant in practice. 

But enough of the pref­ace, pic­tures say more than 1000 words:

Blick auf den Mt. Kil­i­man­jaro, links mit Drop-In, rechts mit Ein­schraub­fil­ter, 16mm, f/6.3

So, what does the com­par­i­son show now? 

Over­all, I see lit­tle dif­fer­ence in both shots. At the same time, how­ev­er, both images show a major prob­lem of all polar­iz­ing fil­ters with super wide-angle lens­es. The effect of a polar­iz­ing fil­ter is high­ly depen­dent on the angle of the light rays to the view­ing axis. Thus, both images show a max­i­mum effect approx­i­mate­ly in the cen­ter of the image, where the light rays come from about 90° from the left. Towards the edges, how­ev­er, the effect decreas­es more and more, since the lens cov­ers an image angle of 108° in the diag­o­nal and thus the angle from the view­ing axis to the light rays devi­ates strong­ly from the ide­al 90°. The edges of the image thus appear sig­nif­i­cant­ly brighter and with less contrast. 

This effect is thus appar­ent­ly inde­pen­dent of whether a drop-in or a screw-in fil­ter is used. Whether you like the effect or not is a mat­ter of taste. But it can­not be avoid­ed when using polar­iz­ing filters.


The Canon Mount-Adapter Plug-In EOS EF-R C-PL allows the use of a polar­iz­ing fil­ter with all EF lens­es, even with those which can­not be equipped with a front fil­ter, like some super wide-angle lenses.

In its effec­tive­ness it is absolute­ly com­pa­ra­ble to a very high qual­i­ty screw-on fil­ter, the light trans­mis­sion is even bet­ter than with my very good B&W Käse­mann polar­iz­ing fil­ter. In addi­tion, by using a neu­tral fil­ter, which can be pur­chased sep­a­rate­ly, the mount adapter can effec­tive­ly pre­vent dust from enter­ing the cam­era, which can be help­ful in dusty environments.

In the near future I plan to buy an addi­tion­al ND-gray fil­ter. So far Canon only offers a vari­able ND fil­ter*, but it is very expen­sive and I don’t need the vari­abil­i­ty. But there are more com­pat­i­ble fil­ters announced by an exter­nal com­pa­ny. I will stay tuned and will report fur­ther if necessary…

Adden­dum: in the mean­time I was able to pur­chase the orig­i­nal vari­able ND fil­ter from Canon at a rea­son­able price. I have already used it sev­er­al times with good results.

*= Affil­i­ate Link

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