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06-17-2013, 11:17 PM   #1
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Going from a "film" to a "digital" back to a "film" lens?

I understand the crop factor when using a film lens on a DSLR with a APS-C sensor. It multiples the focal length of the film lens. The circle imagine cast by the film lens is larger than the sensor size.

Some lenses are designed for digital use and take the sensor size into account. There is no crop factor as far as I can tell.

What happens when these digital lenses are used on a full frame devices, be they digital or film? Will I see lots of distortion since the lens was not optimized? Or, will it work just fine and there will be an image divider this time around?

I ask because if I'm going to sink money into lenses that I want to keep for a long time I want to make sure that the lenses I get today will work on a full frame system.

Thanks!

06-17-2013, 11:39 PM   #2
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focal length is focal length. If a lens is 50mm, no matter the size of the sensor. It will remain 50mm.
If you use it with a APS-C sensor, the FIELD OF VIEW provided by the 50mm is equivalent to 1.5 times the field of view a 50mm lens gives on a full frame camera (same field of view as 75mm lens on full frame camera)
If you use it with a micro 4/3 sensor, the FIELD OF VIEW the FIELD OF VIEW provided by the 50mm is equivalent to 2 times the field of view a 50mm lens gives on a full frame camera (same field of view as 100mm lens on full frame camera)

So, crop factor gives a notion of the EQUIVALENT field of view, if the same lens was mounted on a FF sensor (film or digital)

There are no lenses that take the crop factor into account. If it says 50mm, it will remain that focal length in any format. What does change is the image circle size in order to cover a larger or smaller sensor.
06-18-2013, 12:16 AM   #3
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No, it doesn't multiply the focal length. It gives a 'field of view' equivalent.

A lens designed for APSC is physically smaller, which is one of the attractions of APSC cameras.
It projects a smaller image to fit the smaller sensor size, and will not work correctly on FF.
However, a FF lens will work on an APSC camera, because it only displays part of the projected image from that lens on the the sensor (hence 'crop')

Someone will correct me if I am mistaken, but my understanding is that if you take an image using a FF camera and crop it by 2/3 on the width and height, then that is the image you would get using an APSC sensor.

Again, somebody please correct me if I am mistaken here, but:
I believe people often confuse 'field of view' with 'perspective'.
A 50mm lens approximates to a natural perspective as we perceive it, whether it is mounted on a FF or APSC camera.
Using a 35mm lens (approx 2/3 the focal length) on APSC will not give the same perspective as a 50mm lens on FF, but just a similar field of view.
06-18-2013, 12:25 AM   #4
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QuoteOriginally posted by percy Quote
Again, somebody please correct me if I am mistaken here, but:
I believe people often confuse 'field of view' with 'perspective'.
A 50mm lens approximates to a natural perspective as we perceive it, whether it is mounted on a FF or APSC camera.
Using a 35mm lens (approx 2/3 the focal length) on APSC will not give the same perspective as a 50mm lens on FF, but just a similar field of view.
Perspective is only function of distance to the subject and position relative to subject. Changing Focal length wonīt change perspective.

A 50mm lens approximates to a natural FIELD OF VIEW as we perceive it, when mounted on a FF. Human eye has an angle of view of about 46 degrees with maximum detail. This is the angle of view of a 50mm on FF camera. A 35mm lens on a APS-C camera, has an angle of view of arround 46°. Thatīs why we say it has the angle of view of a 50mm mounted on FF camera.

If you donīt have experience with FF cameras, forget about it. No real use for it.

06-18-2013, 02:27 AM   #5
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Thanks for that clarification
06-18-2013, 02:36 AM   #6
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Crop factor is a function of sensor size and has nothing to do with lenses. Focal length is a physical property of a lens and never changes. The image circle of a lens designed for use on a camera with an APS-C sensor is smaller than the image circle of a lens designed for use on a 135 format camera. As a result, APS-C designed lenses may display varying degrees of vignetting when used on a 135 format camera. As far as I know, this is of little or no consequence for Pentax shooters with digital cameras, as the only 135 format Pentax cameras are film cameras and, last I heard, Pentax has not announced plans to develop and market a 135 format digital camera. (But I don't pay close attention to Pentax news so I might have missed such an announcement.)

Some people have tested APS-C designed lenses on 135 format cameras, and you can find their results by searching the forum.
06-18-2013, 06:41 AM - 1 Like   #7
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there are a lot of myths and half truths in this thread.

First of all, as some have indicated, there is no focal length multiplier, focal length is a physical property of a lens, and if we consider a simple lens for ease of argument, it is the distance from the lens to the focusing plane for the lens focused on an infinitely distant object. in fact, even todays modern lenses with multiple lens elements are only tested for focal length at infinity.

Second, cropped sensors do not necessarily result in smaller lenses, While it is true that film lenses do have a larger image circle (i,e. the diameter of the frame) and this can result in larger rear elements, for any lens longer than the system registry distance, (i.e. the distance from mount to focal plane) the lens size is determined principally by the Maximum aperture (focal length / front element diameter) and the mechanics of the lens barrel and focusing etc. This has been debated long and hard, but if you want a real good indicator, compare a DA300/4 with a K300/4. Front element of the K300/4 and DA300/4 are the same diameter. in fact, even though the K lens uses 100% metal construction, it is actually a hair lighter than the DA. for wide angle lenses, the size is highly variable based upon things other than focal length, and specifically driven by the system registry distance. for pentax this is 45.46mm and for lenses shorter than this, the lens requires a retrofocus group that allows shorter effective focal length for a lens that sticks out the front beyond the camera. As a result for wide angle lenses the mechanics of the system drive lens size, not sensor size

third, while perspective is only a function of position and not focal length, what people forget, is that with a differing field of view between cropped sensor and film, they also change position, this is what leads to the perception that cropped sensors have a different perspective to full frame.

fourth, there are many different lenses designed for digital. some are designed for full frame, some for cropped sensor. with pentax, while they claim the lenses are for Digital (DA) or for full frame (D_FA, FA, F etc) many DA lenses do cover a full frame adequately, there is a thread showing shots taken using DA lenses on film bodies that shows this. The real difference between D_FA and FA lenses is coatings which stop reflections off the rear element and sensor. Sigma lenses marked DC or DG have this coating, the difference between DC and DG is that DG also signifies full frame, DC is copped. Tamron uses Di for full frame and Dii for cropped, but again a cropped lens may have some useful range on full frame. My sigma 10-20 covers full frame from 13mm out.
06-18-2013, 11:56 AM   #8
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Since 6BQ5 posted an almost identical question a few days ago over in one of the digital forums, I have to believe the OP didn't like our answers. And I see a couple of the posters told OP virtually the same thing over in the digital forum.

In short a lens made for a 24mmX36mm film gate will always work on the smaller APS-C sensor. The image projected by the lens is exactly the same on both bodies. On Pentax APS-C bodies the sensor is 15.7mmX23.7mm. If you set the smaller rectangle inside the larger rectangle on the film camera, you will see it covers about 75% of the same area. The inverse of 0.75 is 1.50 - - - this is where the "1.5 crop factor" comes from. If you cut out the center 75% of an old film photo, then enlarge the piece of the photo you cut out to be the same size as the original print, it would be similar to using a lens on the film camera that was 150% longer.

Because that outer 25% of the projected image isn't used on a Pentax digital body, lenses designed specifically for digital bodies often don't worry about what happens in that 25%. Some digital lenses will project a perfectly fine image if used on a film camera, other will show lesser image quality, some will show vignetting in the corners. But the fact remains that the overall projected image size by a lens for a given focal length is always the same, regardless if the lens is designed for a film or a digital camera and regardless if the lens is used on a film or APS-C digital camera. The ONLY difference is how much of that projected image is used by the camera body.

Therefore, focal length is focal length is focal length. The lens properties don't magically change based on camera body.

06-18-2013, 02:45 PM   #9
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Thanks for all the replies! I love all the answers and help that I am getting from everyone here. There's just a lot of information out there and sometimes it's hard for me to absorb it all at once. It's like the old analogy of drinking from a fire hose.

Now I understand that focal length is a physical measurement and is separate from the derived field of view. My focal length of 50mm is fixed and my field of view will change with different camera sensors based on the sensor size. For some reason I had a strange idea that "new" lenses would adjust this number on the zoom ring to give film people an auto-translation of equivalent focal length due to crop based on older film number. Apparently that is not the case! Lesson learned.

I know a lot of P&S cameras try to portray magnification factor vs. focal length. What if cameras tried to portray zoom in terms of field of view in degrees? Would that confuse too many people?

This forum is absolutely huge! There are articles dating back a couple of years and I'm just now starting to find some threads that show how APS-C optimized lenses work in FF film cameras. I must be pretty old fashioned because I really wish I could just find a book at Barnes and Noble that has all this data compiled with glossy photo samples.
06-18-2013, 03:40 PM   #10
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Some of the Pentax's (current) DA series lenses offer a full frame image circle, as well as the D-FA lenses. You can find out which by looking at the DA series in the lens database here, full frame compatible lenses will have a "FF" icon as well as other icons. The FA series are film-era designs so they were made for full frame. Most DA series lenses have a smaller image circle, though, so you will have quite a lot of serious vignetting (blackness), especially at the sides. It's not very usable unless you're going for a 1x1 square crop and are looking for just a bit more focal length than an APS-C sensor will offer.
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