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I have a film SLR that is currently loaded with film. I was using a 28mm lens when I loaded the film but recently got a 50mm and I want to use that instead.

Question is, can I change the lens without exposing the film? I am about halfway through the film so I'm afraid that if I change lens, it will expose and ruin the film.

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The shutter that isolates the film is going to be in the camera body, so changing lenses shouldn't expose the film, unless there is a fault with the shutter (in which case you've probably got bigger problems)... –  forsvarir Jul 13 '12 at 9:16
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Here is an experiment for you: take off the front and rear caps on a lens, hold it up against a light source (NOT the sun!!), and you will notice that you can see right through it. That's how it appears to the camera when mounted, too. As has been said, (in a SLR) it's the shutter (and to some extent the mirror) which protect the film (or sensor) from inadvertant exposure, not the lens. –  Michael Kjörling Jul 13 '12 at 11:00
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3 Answers 3

up vote 14 down vote accepted

Yes, you can. Changing lens won't reveal the film in an SLR; both mirror and shutter are in the way. The lens mount is only meant to pass light to the film during exposure — you can see light through the lens in the viewfinder when composing, but that light doesn't reach the film.

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" is never meant to be passing light only" I'm not sure what you meant there... did you mean to word it that way? –  chills42 Jul 13 '12 at 10:29
    
@chills42 It makes sense to me. If the lens mount was not "never meant to be passing light only during exposure", it would be meant to pass light only during exposure (of the film). If so, you would need a separate viewfinder lens (even if trivial), which goes against the concept of single lens reflex (SLR). –  Michael Kjörling Jul 13 '12 at 10:59
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To put that another way, if allowing light through the lens mount would expose the film, you'd never be able to remove the lens cap. –  Blrfl Jul 13 '12 at 11:01
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@Blrfl Sure you could, you'd just have to be really quick in putting it back on! –  Michael Kjörling Jul 13 '12 at 11:04
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In any camera that you are liable to be using, the shutter — which optically separates the film from the light — is in the camera body, and is closed except when a photo is being taken, so when you remove the lens the shutter will still protect the film.

Long ago there were exceptions - I mention them here for education and 'fun', with no real expectation that you'll actually be using a Kowa, Contaflex or Bessamatic :-).


Related:

In a small number of designs a leaf shutter is used - mounted either in the lens - or behind the lens but in front of the SLR mirror so it must be open to focus and use the viewfinder. In many cases where this is done the lens is not removable (in the usual meaning of the word). Where still used these are now invariably in medium of large format cameras, 'which you have not got'.

If you have an SLR camera with such a lens it will be a Kowa made no later than 1958 or an earlier German camera such as a Contaflex or Bessamatic. (A what?, a who? a ... ??? :-) ).
ie you won't have one. And, if you did, then it would have a "film door" behind the leaf shutter so you could remove the lens without exposing the film. If everything worked as it should :-).

Leaf shutters are what were generally used in Rangefinder cameras - when being closed as a default setting is acceptable - unlike with an SLR when the shutter blades tend to make viewing very very very difficult if closed.

Why then use them in an SLR?
The main advantages of a leaf shutter are synchronisation with flash at any speed that they will work at - typically up to 1/500 th second but 1/1000th in some designs, and lack of the distortion which occurs with high speed objects in focal plane shutters due to the shutter opening moving across the sensor/film partially open so that different parts of the moving subject are imaged on different parts of the sensor/film. Offset against this are very high complexity, higher weight, lower reliability and higher cost. Add to that the need for the complicated dance below and a film door (when film is used) and their death in SLRs was inevitable. FWIW with a DSLR a sensor door would not be strictly necessary.

Looks normal :-) (from this marvellous page

enter image description here Photo copyright 2006 by Frank Mechelhoff

Interest only - here's what happen when you push the shutter button on the above Kowas:

If you press the trigger to photograph, the control circle of a KOWA SET-R(2) shutter includes in the following steps:

  1. closing the shutter blades
  2. closing aperture to the preselected value
  3. mirror upward move
  4. film door upward move
  5. open the shutter again
  6. after preselected time, close the shutter again
  7. closing the film door again
  8. mirror downward move
  9. opening aperture to full value again
  10. opening the shutter again

enter image description here Repair Notices made by Rick Oleson

Kowa with front removed - wow!


Typical modernish leaf shutter:

enter image description here

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Please note, the content on the page linked is clearly identified as copyrighted: "(c) Photos and Text by Frank Mechelhoff". Please remember to get permission and note or link the permission given before replicating copyrighted content. Based on my own personal experience, I'm willing to bet you'll probably find that when you ASK BEFOREHAND, people are usually more than happy to grant permission for use here, however it is still and always appropriate to ask first. –  jrista Jul 14 '12 at 5:53
    
@jrista - I try to always include sources of images. If I fail to do so it's an omission which I apologise for. || And/But: I'd be genuinely interested in knowing how "Fair Use" informs this site's attitude to use of such images. My understanding is that this is exactly the sort of thing that 'fair use' is intended to cover. I may, of course, be wrong. –  Russell McMahon Jul 17 '12 at 3:43
    
@jrista - I note that on another area on SE externally linked images had their links replaced with imgur linked images, without permission or reference to the poster, thereby badly muddying copyright issues. I am NOT saying that I am right, just that the area is complex. –  Russell McMahon Jul 17 '12 at 3:44
    
To be frank, I am rather concerned by the simple volume of copyrighted content that you are replicating here on PhotoSE. I've been through quite a few of your posts, and you are coping a considerable amount of clearly copyrighted content from other peoples sites/blogs/galleries here to PhotoSE, with most of the content uploaded to the SE imgur service. I have never seen any comment or other indication from you that you have requested permission for use, "fair use" clauses or not. For one, its a simple matter of courtesy. Additionally, I am concerned the volume of copyrighted... –  jrista Jul 17 '12 at 4:04
    
...content might eventually cause problems for PhotoSE. The general idea with the Creative Commons CC-by-SA license is that you will grant StackExchange a license to use your own works, with the expectation that if StackExchange replicates any content from any of its sites...say on a blog, or Pinterest, Facebook, etc...that you understand they are being given certain explicit rights to use your content in a certain way. I do not know exactly how the CC license applies in the context of "fair use", however I feel your interpretation of fair use is overly broad in that context. –  jrista Jul 17 '12 at 4:07
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It's exactly the same as DSLR, with the exception that for the latter one you need to be extra careful (and fast) to avoid dirt to be deposited on the sensor.

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Not true. The sensor is usually protected by the shutter and mirror. –  ChrisF Jul 13 '12 at 11:36
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@ChrisF - DSLR 'sensors' get dirty. Dirt gets there with mirror down and shutter closed. Dirt get's into the shutter box 'somehow'. gfunky's comment is essentially true. (Only a D800e get's its sensor dirty - the rest get their low pass filter dirty. The effect s the same.) –  Russell McMahon Jul 13 '12 at 13:39
    
@RussellMcMahon - that may be true but the implication of the answer was the sensor is exposed when the lens is off - which isn't true. –  ChrisF Jul 13 '12 at 13:42
    
@ChrisF - I'm sure we coul;d dance indefinitely on this BUT I read and read his comment quite differently to you, apparently. He said You need to do xxx to avoid achieving yyy. As xxx would indeed achieve yyy, even though there was temporarily a shutter in the way, and mirror perhaps partially in the way then the warning was apposite. ie IF you are NOT careful AND/OR IF you leave the lens off for an extended period THEN you increase the probability of getting dirt on your sensor as a direct consequence. That it may take a mirror wave or few to finalise the result makes no difference. –  Russell McMahon Jul 13 '12 at 15:53
    
@RussellMcMahon A small note: it's a myth that the D800E has no filters in front of the sensor. In fact (for manufacturing reasons) they added a filter to optically cancel the effects of the optical low-pass filter. Even if they removed the low-pass filter, there are still several other filters. The sensor is never exposed, even in medium-format cameras. –  Derrick Coetzee Jul 14 '12 at 0:03
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