Rear-mount filters on Canon 17-40L

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Richard
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#1
Has anyone fitted filters to the rear of a Canon 17-40L?

This photo shows the rear filter slot (it's not mine - I'm not selling! - I borrowed it from here just to show the slot http://i247.photobucket.com/albums/gg149/bpnews88/Personal_Use/Shooting/IMG_3531.jpg ). It's intended for gel filters that you cut up and slide in, but I was thinking of something different.



I am thinking of unscrewing the little clip-mount thing and fitting a glass 10-stop ND filter there, cut to size by those nice people at Lee Filters. I'm thinking of using that sticky-backed magnetic sheet (fridge magnet style) cut into two rings, one stuck on the lens and the other to the filter. The magnetic sheet is very thin, about 0.5mm. That should hold it on okay, seal it from light leaks around the edge, and there is just room for everything to clear the mirror at the back of the lens on my 5D2 - there's about 3-4mm of space.

I can't be the first person to have thought of this, but google doesn't bring anything up. The advantage would be a) you can put a second filter on the front, polariser or grad maybe, without vignetting, and b) because of the way the image is projected from the back of the lens it would dramatically reduce the optical vignetting you inevitably get with a dark ND filter on the front (because a super-wide looks through the side of the filter at an angle towards the edges, making a 10-stop ND a lot darker than that at the periphery).

Apart from whether it's possible or not, I'm worried about reflections coming off the sensor and bouncing back off the rear surface of the filter, it being so close. I wondered if anybody has any experience?
 
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HoppyUK
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#3
downside would be having to take the lens off every time you want to refocus/recompose shot.
Well yes. But if you're setting up for a long exposure with a 10-stop ND, tripod and all, I'm thinking that shouldn't be much of a problem.

The way I work at the moment is I set everything up without a filter, take a couple of test shots to get the settings right, then fit the filter and go.

Of course if you have to keep taking the filter on and off all the time it's going to be a pain, but I know pretty much what I'm doing now and can get it right first time. Not ideal maybe, but workable and there are the other advantages too.
 
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HoppyUK
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#4
Bump :)

Nobody tried filters on the rear?
 
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HoppyUK
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#6
Cheers David. Lee tell me that the only way they can supply a small circular version of their 100mm square ten-stopper is for me to buy one and then pay extra to have it cut - over £100 all in.

So that's not going to happen, but Hoya can supply a 49mm version of their nine stops X400 ND for £55, so I might have one of those cut down. It's coated too.

I'll post how I get on, though it doesn't seem like this great idea has captured the imagination of the whole world just yet :LOL: Cheers!
 
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#8
I`ve never tried it but it sounds like a great idea, I use the circular B+W 10 stopper. I shall watch the thread with interest. (y)
Cheers Ian. Don't hold your breath though ;)
 
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#11
Good idea, if you use live view can it not see through a 10 stop filter. The live view on my 40D can make out an image when I put my welding glass in front of it.
 
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#12
what sort of filters would you put on the rear of a lens?
 
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#13
I actually don't see the point in doing it, personally. It's great to push the boundaries but why not just save yourself time and effort by buying a B&W 77mm 10 Stop filter which will screw in, minimise the risk of light leak, allow you to easily compose, focus and reattach filter and remove the risk of dust getting in to the camera whilst you're faffing around dropping the filter back inside of the lens?

I juts don't understand why you'd want to?


DB
 
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#15
is this a concept from film days and putting WB filters in place?
 
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HoppyUK
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#16
I actually don't see the point in doing it, personally. It's great to push the boundaries but why not just save yourself time and effort by buying a B&W 77mm 10 Stop filter which will screw in, minimise the risk of light leak, allow you to easily compose, focus and reattach filter and remove the risk of dust getting in to the camera whilst you're faffing around dropping the filter back inside of the lens?

I juts don't understand why you'd want to?


DB
You can put any filter you like behind the lens. I explained the particular advanatges with an ND filter in the first post - dramatic reduction in optical vignetting when shooting super-wide the and ability to fit other filters on the front at the same time, which would otherwise cause mechanical vignetting.

These are big advantages as I only seem to use filters for wide-angle landscapes.

The way I am thinking of doing it, using a circular magnetic stick-on pad (and removing the Canon square slot) you could also rotate a polariser or grad if you wanted to, but that would be a bit cumbersome.

I don't think it will be too difficult to use. Just remove the lens, pop it on the back, and it sticks. That's it. Sure that's a bit of a faff in normal photography but when using a dark ND, with all the setting up on a tripod etc that you have to do for that, it doesn't seem too much of a problem.

I already have the B+W ten-stopper.
 
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#17
I know someone who had a welding glass filter cut circular at a local glass place-worth looking into. I dont think you will get problems with light leaks back there, as there isnt any stray light like there is at the front of the lens.
 
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#18
Very interested in this :D

Ive just got a Sigma 12-24 which uses the same filter arrangement.

Ive been thinking about making a grad to use but it occurred to me, Because the amount of graduation increases with distance from the center line, using a full size grad (designed to cover a front element 70mm+) cut down, the amount of graduation from edge to edge is going to be very very slight because the filter has to be chopped down to cover the rear element and your only using a portion of the filter rather than the full range.

In effect to get the same image as a soft 0.9 grad on the front of the lens i would maybe have to use a hard 0.9 on the rear of the lens?

Its difficult to word what i mean lol. Heres a very very basic diagram

The front element with grad will see a reduction across it of.

3ev
2.5ev
2ev
1.5ev
1ev
0.5ev
0ev
0ev
0ev

Where as if the same filter is chopped and put on the rear its only going to see maybe this portion?

1.5ev
1ev
0.5ev
0ev

So using a harder filter on the rear would give a more natural effect (natural in terms of using a normal filter)? Or do they make filters with more abrupt transitions for use with smaller lenses?

Of course you wont have this issue with an ND but be interested to hear anyones thoughts on using grads
 
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#19
I actually don't see the point in doing it, personally. It's great to push the boundaries but why not just save yourself time and effort by buying a B&W 77mm 10 Stop filter which will screw in, minimise the risk of light leak, allow you to easily compose, focus and reattach filter and remove the risk of dust getting in to the camera whilst you're faffing around dropping the filter back inside of the lens?

I juts don't understand why you'd want to?


DB
You could mount a filter in the back and still use a circular filter on the front, without the risk of vignetting. I could see this being very useful in certain situations to be honest. :)
 
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HoppyUK
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#21
You could mount a filter in the back and still use a circular filter on the front, without the risk of vignetting. I could see this being very useful in certain situations to be honest. :)
You could do it with the welding glass just as well I think. That glass is quite thick and there's not much room at the back though.

I'm not sure about light leaks, which is why I like the magnetic sticky bit - seals it. While there is certainly nothing like the amount of light blasting around like there is on the front, the light hitting the front of the filter, and potentially reflecting off it, is still one thousand times brighter than the image-forming light that comes out of the back. Ten stops is 1024x and most welding glass is darker than ten stops so more like 2000-16000x.

Edit: welding glass is also heavier, being thicker. There is not that much 'stick' with only a small circle of magnetic material and you don't want it coming off :eek:
 
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#22
I'm not sure about light leaks, which is why I like the magnetic sticky bit - seals it. While there is certainly nothing like the amount of light blasting around like there is on the front, the light hitting the front of the filter, and potentially reflecting off it, is still one thousand times brighter than the image-forming light that comes out of the back. Ten stops is 1024x and most welding glass is darker than ten stops so more like 2000-16000x.
true, i was forgetting the significance of it been welding glass and that there would be large amounts of light coming through the lens, but little beyond the filter.
 
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#23
true, i was forgetting the significance of it been welding glass and that there would be large amounts of light coming through the lens, but little beyond the filter.
Thinking about this a little more, the problem with a front mounted dark ND in a regular slot-in holder, is light creeping round the back of the filter, and in through the edges too I think.

That wouldn't be a problem with a rear mount so maybe it's not such an issue after all - could do it with BluTack! But I think the magnetic thing is quite a good solution anyway, so I'll stay with that plan.
 

jgs001

Brian Cox
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#24
There are specific astro filters (EOS Clip) as far as I know only made for Canon, that clip into the body itself, in front of the mirror box. I'll see if I can find a piccie, if not I'll take one showing the design... it might be something you can use... that way you're not reliant upon modding the lens.

It's not a great pic... on here.. That's the LP variant... they do a clear one for £65...

http://www.telescopehouse.com/acatalog/Astronomik_CLS_Filter___Canon_EOS_clip_fit.html

I'll take a more face on shot of my LP clip filter later and post it up for you.
 
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#25
would the weliding class fit in the slot. i.e thickness?
 
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#26
Very interested in this :D

Ive just got a Sigma 12-24 which uses the same filter arrangement.

Ive been thinking about making a grad to use but it occurred to me, Because the amount of graduation increases with distance from the center line, using a full size grad (designed to cover a front element 70mm+) cut down, the amount of graduation from edge to edge is going to be very very slight because the filter has to be chopped down to cover the rear element and your only using a portion of the filter rather than the full range.

In effect to get the same image as a soft 0.9 grad on the front of the lens i would maybe have to use a hard 0.9 on the rear of the lens?

Its difficult to word what i mean lol. Heres a very very basic diagram

The front element with grad will see a reduction across it of.

3ev
2.5ev
2ev
1.5ev
1ev
0.5ev
0ev
0ev
0ev

Where as if the same filter is chopped and put on the rear its only going to see maybe this portion?

1.5ev
1ev
0.5ev
0ev

So using a harder filter on the rear would give a more natural effect (natural in terms of using a normal filter)? Or do they make filters with more abrupt transitions for use with smaller lenses?

Of course you wont have this issue with an ND but be interested to hear anyones thoughts on using grads
I understand what you're saying. TBH I don't think the idea of sticking a grad behind the lens is a practical plan, relative to how easy it is to use one on the front. But if you did, it should overcome (or reduce substantially) the effect of additional darkening away from the centre with a super-wide lens.

It would also change the effect of the grad, making the line softer and possibly missing the full darkening effect simply because the image would never actually get projected through it. Your estimate of a hard-cut producing a similar effect to a soft-cut is probably not a bad guess. It's the same effect as you get using longer lenses with a grad - the grad line is substantially softened and the darkening reduced, almost to the point of becoming invisible with very long lenses. They might have a 77mm fitted on the front, same as a super-wide, but they only look through a small part in the centre.

Bear in mind that you can also vary the effect of a grad quite a bit with aperture - high f/numbers make them darker with a harder grad line. It's a depth of field effect. By the same token, the effect is varied depending on how far from the lens surface the filter is mounted.

Final point. When a lens is only using a small section of the filter, any imprefections are magnified. This is why cheap filters on long lenses sometimes create sharpness problems. Going back to the idea of fitting the welding glass ND behind the lens, this would be a concern. It's not brilliant on the front! (But it has to be said, not at all bad for a couple of quid either.)
 
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#27
There are specific astro filters (EOS Clip) as far as I know only made for Canon, that clip into the body itself, in front of the mirror box. I'll see if I can find a piccie, if not I'll take one showing the design... it might be something you can use... that way you're not reliant upon modding the lens.

It's not a great pic... on here.. That's the LP variant... they do a clear one for £65...

http://www.telescopehouse.com/acatalog/Astronomik_CLS_Filter___Canon_EOS_clip_fit.html

I'll take a more face on shot of my LP clip filter later and post it up for you.
That's an interesting idea (y) Won't fit full-frame though - mirror gets in the way.

would the weliding class fit in the slot. i.e thickness?
No chance. It's designed for gelatin filters which are a paper thin. You could extend the gap easily enough I think, with longer screws and a spacer, but you need to bend the filter slide it in. That's one reason I decided against it.
 
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#28
well i am due to get one of these soon and just looking at the options... cant see me using the rear filter much unless i got a custom 10 stop filter. i mainly just use a CPL or a UV depending on enviroments.
 

jgs001

Brian Cox
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#29
Ah well sorry Richard, I guess this won't be a lot of use ... but in case anyone else might find it useful...




it's not the best of shots :D... grabbed with the Z2 as it was handy...
 
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#30
what exactly does that filter do?
 

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Brian Cox
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#31
That particular one is a light pollution suppression filter. It was more the in body clip in mounting...
 
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#32
Ah well sorry Richard, I guess this won't be a lot of use ... but in case anyone else might find it useful...




it's not the best of shots :D... grabbed with the Z2 as it was handy...
Thanks for that John. Very interesting - I never knew such a thing existed. Just a shame it won't clear the full frame mirror, though I'm guessing it might also clip the edges of a full frame image also.

Just for information, how does it fit? Is it easy to clip in and out? How thick a filter could you attach in that way before the mirror causes trouble? I'm assuming the camera and lens work completey normally - normal metering would obviously be buggered ;)
 

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Brian Cox
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#33
I guess it might clip the edges... As for fitting, it just slots into the hole in the body... I'll set it up at some point in the body and take a shot. I'll also do some measurements on it too. It's fine with EF lenses

http://www.talkphotography.co.uk/forums/showthread.php?t=196647

the image in that thread was shot with this filter and the Nifty Fifty, EF-S lenses stick too far into the body and the read element collides with the clip filter. As for metering... I've never tried... no point when you're aiming for 5 minute or longer exposures ;)...
 

jgs001

Brian Cox
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#34
Richard, Here's a shot with it in situ



the clear area is 27mm x 24mm, the thickness is 2mm
 
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#35
Richard, Here's a shot with it in situ

<snip>
Thanks John. Might be something there for crop camera users. And thinking about it for a moment more, metering and AF etc should work fine - at least in theory. If it doesn't block the viewinder image.
 

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Brian Cox
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#36
It's a shame the clear filter is £65 or I'd get one to use with my scope (when not using it for Astro imaging) to keep the dust out of the body.
 
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#37
How does it clip in assuming it does ? Maybe a glass from an old fashioned slide mount could replace the coloured glass in your holder. Just measured one at about 38 x 26 and less than 2mm thick. Just a thought.
C
 
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