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10-27-2010, 03:55 PM   #31
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Jsherman999

Although we didn't have sensors and pixels we had film speeds different emulsions and grain

Media resolution was always an issue even in film days

10-27-2010, 04:08 PM   #32
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The technical definition of focal length is the distance from the optical center of the idealized lens to its focal point. Photographic lenses are obviously non-ideal lenses, being made of many individual lenses (elements). Thus it is very possible to have the "optical center" be physically located outside of the lens itself (most long telephoto lenses are like this). However, the principle still holds: focal length is just a way of indicating how strongly the lens system bends the light, so as others have said "50mm is 50mm" no matter if it's DA, FA or whatever.

All this talk of "crop factor" can be rather abstract to some people, so allow me to "borrow" an image from Wikipedia which I think is the best demonstration as to what the "crop factor is":



The red box is the FF's field of view, and the blue box is APS-C. The image made by the lens remains the same. It's just how much of that image is recorded by the sensor that makes the difference.
10-27-2010, 04:22 PM   #33
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QuoteOriginally posted by RioRico Quote
Resolution and DOF and perspective remain the same.
Close, but no cigar...

There is one qualifier to this statement. The assumption is that the position of the lens relative to the subject and sensor/medium remains the same. Unfortunately, to frame the subject the same (i.e. get the same picture), we have to change the position of the lens. That changes all of the above and is the cause of most of the confusion.

I have taken part in literally dozens of this type of thread since joining PPF and can cook the pertinent factors down to a few discussion points:
  • Focal length is focal length regardless of frame (sensor) size. It is a matter of physics. Live with it.
  • Perspective is based on the position of the lens relative to the subject and sensor/medium. There is no such thing as tele or wide-angle perspective. Again, this is a matter of physics. Live with it.
  • Image circle is not the same as field of view nor is it dependent on it
  • Field of view is not the same as image circle nor is it dependent on it
  • Field of view is based on both focal length and frame size. For a given frame size a longer focal length will yield a narrower field of view. Similarly, for a given focal length, a smaller frame size will yield a narrower field of view.
  • For a given final image size (screen or print), a smaller frame will require "more" from the lens to deliver the same apparent sharpness as from a larger frame. That is why many vintage lenses when applied to APS-C come up short.
  • DOF is something that you really don't want to get into a discussion on. It is probably enough to say that it is related to absolute aperture (not f/stop), magnification (not focal length) and the notion of "acceptable sharpness". Damn the circles of confusion!
  • Wide angle lenses do not have greater DOF
Let me also add that coming to grips with the above points was a learning experience that continues to this day. I have recently started doing large format (view camera) film work and understanding of the above is "stock-in-trade" in the LF world and working in multiple formats with cameras that have movements has helped immensely in cementing some of these concepts.

Well, there is enough fodder in the above set of bullets to keep this thread going for another month or so. My work here is done! <evil laugh>



Steve
10-27-2010, 04:27 PM   #34
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QuoteOriginally posted by Lowell Goudge Quote
Media resolution was always an issue even in film days
...And that is why it is more important to talk about system resolution rather than simply lens or sensor.


Steve

10-27-2010, 04:47 PM   #35
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Question 2

Just wondering.. why then would a company like Pentax not make a 70-200 (for eg.) to have the same standard FL as the FF versions and instead makes a 50-135 to match the FoV of the FF?

Sort of like answering my own question.. I remember reading somewhere that Pentax wants to give it's FF users the same FoV on their APS-Cs when they migrate to the digital ones. But, just having only equiv FoV is just a feel-good factor. Even so, would they not have to face a change in the working distance (albeit little)?? This in my view is more a practical, physical and perceivable change to the photographer compared to a change in the FoV.. more so for a photographer into numbers.
10-27-2010, 04:52 PM   #36
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QuoteOriginally posted by jsherman999 Quote


This seems like a 'duh' concept, but it kinda makes the "a 50mm lens is a 50mm lens" one of those statements that's technically correct, but has little bearing in real world applications (unless you only ever shoot one format, then it does, and the crop factor discussions become annoying. )


.
How about those of us who shoot or at least understand 5 or more formats and still find crop factor discussions annoying.
Again, focal length is focal length is focal length.
Any discussion about what is behind the lens is a canard in discussing focal length.
It was this way with film, and digital hasn't changed anything, even though people seem to think it has.
10-27-2010, 05:22 PM   #37
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It is a good thing to tell users both, the
- actual focal length, and the
- 35mm equivalent focal length.

Not everybody cares about sensor size. But everybody cares about what photos can be made. Imagine the confusion if P&S cameras wouldn't mention their equivalent FL... Hey, it's even in the EXIFs...

New users just have to know that markings on an SLR lens always are the actual FL.


To add to the confusion, actual aperture and 35mm equivalent aperture differ by the same factor the FL does.

Last edited by falconeye; 10-27-2010 at 05:31 PM.
10-27-2010, 05:47 PM   #38
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QuoteOriginally posted by falconeye Quote
It is a good thing to tell users both, the
Not everybody cares about sensor size. But everybody cares about what photos can be made. Imagine the confusion if P&S cameras wouldn't mention their equivalent FL... Hey, it's even in the EXIFs...
Not that it should not be mentioned, but how many P&S users (and that's a HUGE number) really care what the equiv is?

QuoteOriginally posted by falconeye Quote
It is a good thing to tell users both, the
To add to the confusion, actual aperture and 35mm equivalent aperture differ by the same factor the FL does.
That just went over my head.. but brushed a bit! I know that a given aperture (which is the ratio of FL & aperture diameter) of a given lens remains constant for different sensor sizes. So, what does that mean from the end-image perspective? What changes?

10-27-2010, 06:06 PM   #39
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QuoteOriginally posted by jsherman999 Quote
.

Because, like it or not, pixel density and sensor resolution play a large role in things now, in people's choices, and that's pretty much tied directly to cost.

I want a certain FOV; I can shoot a 450mm f/5.6 lens on FF, or a 300mm f/4 lens on APS-C. They both will give me roughly the same image (using the same pixel density,) but the FF solution will cost probably 3 times as much.

So, I could just use the 300mm lens on the FF body and crop the image to save lots of money, but then I'm down to 5MP. Big enough to print at 11x16? Maybe. Some would say, not. And what if I want to print larger, or sell the image to someone who might, or crop the image further? Would a TC be better? (surprisingly, in many cases, no.)

And if I'm cropping most of my FF telephoto images, would it make sense to pick up a used APS-C body for my telephoto needs, and get all those pixels inside the image circle? By George, it might!

But what if I put that APS-C money towards a higher-resolution FF body? Will 10MP in the image circle be enough after a crop?

(and the discussion continues, with opinions mixing with biases mixing with myths... )


.
So where does the lens magnification come into play? A 450mm lens as I understand it presents an image 9x mag where the 300mm would be 6x mag.

Surely this would make the subject image size different (on sensor)?

Just that I didn't notice this mentioned.

Best regards
10-27-2010, 07:07 PM   #40
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QuoteOriginally posted by icypepsi Quote
I know that a given aperture (which is the ratio of FL & aperture diameter) of a given lens remains constant for different sensor sizes. So, what does that mean from the end-image perspective?
To make equivalent images you need different FLs on different formats. When you change the FL you need to change the f-ratio accordingly, otherwise you'd change the aperture diameter which would result in different DOF, i.e., not an equivalent image.
10-27-2010, 08:37 PM   #41
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Here is an example of the differences..Both shots were taken standing in the exact same spot.

28mm lens on 35mm film camera.



28mm lens on K110D DSLR

10-27-2010, 10:00 PM   #42
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QuoteOriginally posted by Stratman Quote
Here is an example of the differences..Both shots were taken standing in the exact same spot.
Steve...not wanting to be critical, but are you sure those were both taken from the same spot? The perspective is quite different between the two photos. The camera was quite a bit closer to the caboose in the second photo. (See the relative positions of the tree tops at the left top of the roof and the line of cars behind the fence to the right.)


Steve (the other)
10-28-2010, 12:34 AM   #43
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QuoteOriginally posted by falconeye Quote
It is a good thing to tell users both, the
- actual focal length, and the
- 35mm equivalent focal length.
To be precise and correct, 35mm equivalent focal length should be worded as 35mm focal length producing equivalent FOV. But the latter would generate confusion at the point of sale, so a simpler term is provided and the customer has to deal with the issue on forums.
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