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05-22-2010, 08:15 AM   #1
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35mm / APS-C -

As far as I know a 35mm focal lenght in the old 35mm film format should be equivalent to a 50 mm in the APS-C format used by my K7...
That said...
I attach two shots,
- the first one has been made with the Pentax DA 18-55 AL WR at 35mm, this is the zoom lenses that came with my K7
- the second one has been made with the Pentax FA 20-35 AL at 35mm, this is a several yars old lenses that I used with my old Pentax MZ5n, a 35mm camera

I expected that the photo taken with the "old" FA 20-35 was much "tele" that the one taken with the DA 18-55... but as you can see the snapshots are practically identical...
The same happens with the DA55-200 vs my old Tamron SP 70-210... at max focal length they take the same telephoto... while I expected the tamron to have a much narrow angle...

how do you explain that ?

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PENTAX K-7  Photo 
05-22-2010, 08:39 AM   #2
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That's because you're understanding is wrong. 35mm is 35mm, regardless of whether the lens was made for FF or APS-C. Two different 35mm lenses *on the same camera* will produce the same FOV. But they'll produce different images *on different cameras*. 35mm mounted to a FF camera will produce a wider view than 35mm mounted to an APS-C camera. Said another way, the so-called "crop factor" is a way of comparing *cameras* when using a given *lens*, not a way of comparing *lenses* when using a given *camera*.

I'm moving this thread to the Beginner's forum because it is an extremely common point of confusion among newcomers to the DSLR world.
05-22-2010, 08:42 AM   #3
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They look the same because both lens were at a focal length of 35mm on the same body. The actual focal length of a lens doesn't change. What changes is the angle of view when using for example an FA 35mm f2 lens on an MZ-3 film body and then putting it on a K20d dSLR body. That's because the aps-c sensor has smaller dimensions than the 135 film (aka 35mm film).

Edit: Marc posted while I was typing.
05-22-2010, 08:43 AM   #4
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35mm is 35mm. Period. It doesn't matter whether it is a FF lens or not. What "a 35mm focal lenght in the old 35mm film format should be equivalent to a 50 mm in the APS-C format" means is that a picture taken on a FF or 35mm camera with a 50mm will look (nearly) the same as one taken on APS-C with a 35mm lens.

05-22-2010, 09:03 AM   #5
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You've basically asked why 2 lenses of equal focal length, on the same camera produce images with the equal field of view.
05-22-2010, 09:10 AM   #6
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It never ceases to amaze me how much x1.5 confuses people. It's absolutely remarkable.
05-22-2010, 09:19 AM   #7
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35 mm is 35 mm. The difference is that the cropped sensor only uses the central portion of the image circle. With a lens designed for APS-C, the image circle is often smaller in the first place and not useable on a larger sensor or film, but it is still the same focal length.

For example, the FA 35 which covers a film sensor will look the same as the DA 35 which doesn't as far as field of view goes. The camera companies sold crop sensors as making focal lengths longer, but in reality what they do is just crop and magnify the image.
05-22-2010, 09:40 AM   #8
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QuoteOriginally posted by Alfisti Quote
It never ceases to amaze me how much x1.5 confuses people. It's absolutely remarkable.
It never ceases to amaze Me that people actually care.



05-22-2010, 10:11 AM   #9
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Put that same lens on the film camera at the same position and see what you see
05-22-2010, 11:35 AM - 2 Likes   #10
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QuoteOriginally posted by Alfisti Quote
It never ceases to amaze me how much x1.5 confuses people. It's absolutely remarkable.
It never ceases to amaze me how condescending some "pentaxians" can be to people simply trying to learn. Thank Zeus that the vast majority of experienced photogs on this forum know that it is here to educate and inform anyone and everyone regardless their level of experience. Snobbery alienates.

Had to say something!
05-22-2010, 09:41 PM   #11
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What amazes me is the proliferationby the photo media in the whole issue. You would think After 7 years they would stop discussing and comparing it
05-22-2010, 11:17 PM   #12
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First thanks to all of you for the answers.

I know that a given focal length lense produces the same image on both cameras, the problem here is that the measuring unit: focal length on the DA lenses are expressed in their 35mm eqivalent ?
Yes, else there's no way to explain the behavior.

Or Am I wrong ?
05-23-2010, 12:20 AM   #13
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QuoteOriginally posted by sophotec Quote
I know that a given focal length lense produces the same image on both cameras, the problem here is that the measuring unit: focal length on the DA lenses are expressed in their 35mm eqivalent ?
No. Lenses are always labeled with their actual focal lengths.

QuoteQuote:
Yes, else there's no way to explain the behavior.
Of course there is - everyone on this thread already did. A 35mm lens is a 35mm lens, regardless of whether it was made for FF or APS-C. It is labeled 35mm, and it acts like a 35mm lens. All 35mm lenses produce the same FOV on any given camera. This is exactly what you are seeing.
05-23-2010, 02:36 AM   #14
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QuoteOriginally posted by sophotec Quote
First thanks to all of you for the answers.

I know that a given focal length lense produces the same image on both cameras, the problem here is that the measuring unit: focal length on the DA lenses are expressed in their 35mm eqivalent ?
Yes, else there's no way to explain the behavior.

Or Am I wrong ?
There's no differences in 35 mm on either of the lenses. They focus the light the same. On a lens for a film frame the resulting image circle has been made to be big enough to cover the sensor, on some lenses for digital systems (rather read smaller sensors) they don't care so much for the edges since the sensor isn't that large anyway and everything outside will be cropped away anyway.

If you had mounted your Pentax FA 20-35 AL on a full frame body the resulting image circle would had been bigger than your current sensor and covered the whole film frame, and since that film frame is wider than the aps-c sensor in your current camera you would had got wider coverage on the garage of whatever it is. And you'd see more along the sides of the building.

But now since your aps-c sensor is smaller you only get the center of the image and regardless that your FA lens can draw a bigger image circle only the middle of it is used, and hence you only see the garage and not much out on the side.

Mount the DA lens and it still cover your whole sensor with light so it look the same.




I don't know how it look since I have no DSLR or designed for digital lenses but I assume that if you would mount the DA glass on a full frame body you'd get vignetting on the sides, or maybe not big enough image circle with totally dark edges, or maybe just less sharpness around the edges since it's not designed for good image quality so far out since there's not sensor catching that light anyway.




This would had been much easier to draw.

Even point'n'shoot cameras is speced with their real focal lengths, look at this Pentax Optio P70 for instance:


4.9-19.6 mm.

That would be very wide on a full format body but the sensor in that one is very small so it will only catch the center which would crop away lots of information along the sides so even if you'd be able to catch your whole garden with a full frame 4.9 mm on such a small sensor only your garage may be left.

(I hope I don't mess anything up now since the lens for the point'n'shoot isn't designed for the same lens to film/sensor distance, but the idea is the same anyway.)
05-23-2010, 03:13 AM   #15
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Or think of it this way:


You get a black screen of some sorts and cover your window(s) in a room with it and cut a small hole in it for light to pass by, you then shut all the doors and turn of any light so your room get completely dark.

You then go fetch a canvas frame and hold it at some distance from the cut hole in the screen and let it catch the light falling in rendering the scenery outside.

Now say you filled in any lines / draw that scenery / it was stuck on the canvas frame.

Now get a knife and cut away a smaller rectangle only from the center of your canvas frame.

Now if you look at that smaller cut from the frame it won't have as wide coverage of the scenery outside as your whole canvas frame did, right?


The dark screen on your window is still the same and the light fall in at the same angles and so on, but your crop have resulted in an image coverage which is different from what you had on the bigger frame. To get the same result using the bigger frame light must had fallen in at a much narrower angle (and if you cropped a piece of that canvas frame you'd end up with an even narrower result, and so on..)






That's why the crop will look narrower with the same glass compared to what it would had done on a bigger frame. But use the same glass (35mm) and you'll get the same result.
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