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09-12-2007, 02:42 AM   #1
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DSLR lens strangeness

Hi all. New to the forum, so apologies if I don't follow etiquette or am going over something which has already been discussed (I did search first honest!). Also I hope it is OK to discuss the Samsung GX-10 as it is of course the same as the K10D.

I am new to DSLR photography and have just started a course. One of the first things we learned was about the focal length multiplier and how that affects the focal length of our lenses and how they compare to 35mm film equivalents. We learned what was a standard lens (50mm FL) and how on a DSLR, standard is more like (35mm FL). I am the only one in the class with a Samsung and I discovered that standard on my lens was about 50mm despite the 1.5x multiplication factor.

So, I did some tests using my kit 18-55 D-XENON and an older 28-80 film lens from my old Pentax MZ-5N. Setting both lenses to 28mm, the two pictures attached are almost identical. My understanding is that the digital lens should appear closer than the same focal length on the film lens because of the image circle it generates?

Has anyone else come across this or am I missing something due to my lack of understanding? It seems to me that the D-XENON lens (and presumably the Pentax equivalent is the same) are marked with their film equivalent focal lengths? The D-XENON lens definately isn't a film lens as the image circle is smaller - I put it on my MZ-5N and there is clear vignetting in the view finder when at the widest setting.

Does this perhaps explain why my kit came with both the 18-55 and the 50-200? Perhaps the next test I should do is to take the same photograph with the same apparent FL on two different cameras and compare? I would expect to see mine being wider, right?

Any thoughts gratefully accepted.

Many thanks

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09-12-2007, 03:00 AM   #2
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Hi there and welcome.

It's perfectly normal that you'd get the same picture from zooms set at the same focal length on the same camera.

Field of view for a given focal length is a function of the sensor size (ie the camera) whereas field coverage only relates to the ability of a given lens to "illuminate" a given surface.

If you were to test your old lens on a film camera and on a digital camera, you'd get diferent fields of view because the APS-sensor is smaller than the 24x36 film, you'd have to crop the 24x36 image by 1,5 to get the same results as the APS sensor for a given focal length.

On the other hand, if you were to test your digital lens on your 24x36 setup, you'd find that the corners of the picture would be dark (ie vigneting) because the DA lense has a smaller image circle.


Hope this helps.
09-12-2007, 03:05 AM   #3
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The crop factor exists because the size of the sensor in the digital camera is smaller than 35mm film. It doesn't have anything to do with a lens made for a digital camera versus an old lens made for a film camera.

Also, regarding this sentence:

"One of the first things we learned was about the focal length multiplier and how that affects the focal length of our lenses and how they compare to 35mm film equivalents."

The "focal length multiplier" does not affect the focal length of your lenses. A 35mm lens is a 35mm lens. However, because of the smaller size of the sensor vs a frame of 35mm film, a lens with a focal length of 35mm used on a digital camera will have a field of view approximately equivalent to the field of view of a 50mm lens on a film camera.

Hope this helpful to you, and happy shooting.
09-12-2007, 03:13 AM   #4
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One other thing--you should encounter no problems using a "film" lens on your digital camera, but as you noticed, you can't really use a "digital" lens on a film camera. Lenses made for the digital cameras do indeed have a smaller image circle, as the image circle only needs to cover the area of the smaller sensor.

09-12-2007, 03:18 AM   #5
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There are two things that determine the field of view (i.e. what you see as the picture) of a lens, Focal length and sensor/film size.

Focal length is an opticial property of the lens, kind of like diopters on your glasses or contacts, a 28mm lens is a 28mm lens whether it is designed for film, digital, video, whatever. The individual parts of the image projected by a 28mm lens will be the same size no matter what the lens. Lets say you are taking a picture of your house, perhaps the distance from the ground to the tip of your house is 30 feet in real life but in the image projected by a 28mm lens that distance is 1 cm.

So you have a 1cm tall house in an image, now we get to the other factor that determines field of view, the size of the sensor/film, the sensor on your dSLR is about 1.7 cm high so you will have an image with 1 cm of house and .7 cm of sky, regular 35mm film is 2.4 cm high, medium format and large format are larger still (hence will include more sky in the picture).

You mention 2 other terms which need to be explained crop factor and "image size".

Crop factor is simply the amount by which your sensor is smaller than regular 35mm film, someone used to regular film will know about what the field of view his 28mm lens is, however becasue of the smaller sensor the field of view will be smaller so this 28mm lens will "feel" more like his old 40mm lens. Along those same lines someone swiching ftom 35mm film to a medium format will find that a 75mm lens on a medium format camera feels more like a 50mm lens on thier old 35mm camera. If you've never used a 35mm SLR before crop factor means nothing to you, your 28mm lens will do what is does and you don't really have anything to compare it to.

You also mention image size, image size is not the size of the individual elements of the image but rather the size of the circle of light that your lens projects, yes your kit lens has a smaller image size, this means that if you were to mount it on an older 35mm camera (or a full frame dSLR) and set it at 28mm your house would still be 1cm tall but the corners of the image would be black becasue the lens is not projecting an image large enough to cover the entire piece of film/sensor.

Edit: And hey look in the time it took for me to type that up 3 people have already replied
09-12-2007, 04:11 AM   #6
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As others have said, both film and digital lenses will act the same way at the same focal length on the same camera as far as the focal length multiplier is concerned. The focal length multiplier is a factor of the camera; the distance from the lens to the internal sensor or film plain. Therefore, to see a difference, you would need to place one lens on a film camera and the other on a digital camera, set both to the same focal length number, and then compare the results.

However, disregarding that (and keeping this very simple so as to not overwhelm you with too much info), there are slight differences between a digital and film lens when it comes to how well they focus on that sensor or film plain. Film lenses were designed to focus on a point where film is located in a film camera, which several millimeters behind the point where digital sensors are located today in digital cameras. Hyprid lenses (those supposedly optimized for both film and digital) split the difference and focus at a point midway between a digital sensor and the old film location. The new digital lenses give up on film entirely and focus solely on the location of digital sensors today.

Of course, you're not going to see any of this in your images because it's only a matter of millimeters, with any minor focusing differences well within the depth of field of a particular lens and camera combination. Someone running tests with a focusing chart using a very narrow depth of field might notice it, but it very seldom matters in the real world.

However, a digital lens often does benefit a digital camera since this combination can often focus more quickly and more accurately. This is caused by the fact that the len's focus point is directly on the digital sensor (clearest and sharpest at that point, hence the camera can find it quicker) and the internal components inside a smaller lens usually travel less distances to reach the focus point. A side benefit to you is that a digital lens can be built shorter and lose some of the excess glass once used for the larger 35mm film. In other words, shorter, smaller, lighter, lenses which tend to focus slightly quicker on a digital camera.

stewart
09-12-2007, 04:46 AM   #7
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Thanks for all your replies. I think maybe I am confusing myself. Is it right then that I cannot compare a film lense and a digital lense on the same camera? The field of view that people talk about would only become apparent if I took the two photos on the different media, i.e. film and sensor? Its not directly the affect the lens has on the image being projected through it, but what effect the camera has on the image as it is captured - i.e. the digital sensor would capture less (because it is smaller) and therefore when the resulting image is enlarged to the same size, the subject would appear closer?

What really started it off was the comment by my lecturer about what was a 'standard' lens, i.e. 50mm on a 35mm camera was considered standard, but on a digital one, 35mm was nearer to standard. When I looked through the VF, it seemed to me that 50mm was nearer the mark on my camera, the lecturer took a look, agreed and thought it was a bit strange.

Picking up what one reply said about 'feel', does this only apply if I took the two photos on different cameras and then enlarged the resulting photograph to the same size, the digital camera image would appear as though it was taken with a longer lense on the film camera?

I guess I am most concerned that what I see through the vf is pretty much what I am going to be capturing on the senser.

Many thanks
09-12-2007, 05:19 AM   #8
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QuoteOriginally posted by fredd500 Quote
What really started it off was the comment by my lecturer about what was a 'standard' lens, i.e. 50mm on a 35mm camera was considered standard, but on a digital one, 35mm was nearer to standard. When I looked through the VF, it seemed to me that 50mm was nearer the mark on my camera, the lecturer took a look, agreed and thought it was a bit strange.
What should concern you is that your lecturer doesn't understand a concept as basic as focal length of a lens. It is a tricky idea for people new to photography, but anyone proclaiming to be a photography instructor should know it right away.

09-12-2007, 05:26 AM   #9
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By the way, this was discussed in regards to 6x7 lenses here:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/9459-k10d-6x7-lenses.html


You can tell your instructor that on a 6x7 camera, that same 50mm lens becomes an ultra-wide.
09-12-2007, 08:55 AM   #10
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I think perhaps it is me who is confusing the words and unfair to blame my instructor. I suspect what I mean is field of view rather than focal length multiplier.

I found a useful page on dpreview which seems to explain it quite nicely (for me at least)!

Focal Length Multiplier: Optical: Glossary: Learn: Digital Photography Review

Many thanks for all your replies.
09-12-2007, 06:27 PM   #11
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QuoteOriginally posted by stewart_photo Quote
However, disregarding that (and keeping this very simple so as to not overwhelm you with too much info), there are slight differences between a digital and film lens when it comes to how well they focus on that sensor or film plain. Film lenses were designed to focus on a point where film is located in a film camera, which several millimeters behind the point where digital sensors are located today in digital cameras. Hyprid lenses (those supposedly optimized for both film and digital) split the difference and focus at a point midway between a digital sensor and the old film location. The new digital lenses give up on film entirely and focus solely on the location of digital sensors today.
This is not true. The focal plane distance for a given mount type is the same whether it is film or digital. i.e. the focal plane distance from the mount is the same for ALL K-mount lenses and cameras.

The difference between film and digital lenses is:
1) Digital sensors are highly reflective, and this can cause problems with lenses that do not have good antireflective coatings. Thus "optimized for digital" really only means "we didn't skimp on our multicoatings like we did for our previous lenses" (For third party lenses this matters, for Pentax lenses it doesn't really since with a few exceptions they are all SMC, which is more than sufficient for this need and why old Pentax SMC lenses work just great on Pentax DSLRs, at least in terms of optical performance.) Third-party designations for this category are Di for Tamron and DG for Sigma. They are listed as "for film and digital" because they have the necessary multicoatings for digital cameras (which do not penalize them on a film camera, and in fact help them) but retain image circle coverage necessary for 35mm film.

An example of an old pre-digital lens that did NOT have sufficient coatings is the first generation Sigma 28-200 3.8-5.6 - It performed fine for years on my family's old PZ-70, but had abysmal contrast when used on a K10D.

2) Digital sensors are often smaller (APS-C is common, but most other manufacturers have multiple sensor sizes available). As a result a lens can be made that has a smaller image circle coverage but still fully covers the sensor. This allows a lens to be lighter and smaller than it normally would be for a given focal length/aperture. (See, for example, the DA 50-200, which is incredibly light and small.) Designators from third-party vendors for these "digital only" (which really only means "small frame only" lenses include DC for Sigma and "Di II" for Tamron.

The focal length and aperture of the lens specified will never be modified from actual values, since this is an inherent property of the lens common across all cameras it might be mounted to. On Pentax cameras, all DSLRs happen to be 1.5x crop factor currently , but other systems have a couple of crop factors available that are body dependent (For example, I think Nikon has a few different sensor sizes that range from 1.3 to 1.6). It wouldn't make sense to put a body-specific specification on a lens that might be used with a different body, even in a case like the Pentax DSLRs where it looks like they will be APS-C for a long time to come and never anything else.

The only time you will see "35mm equivalent" focal length specs put onto a camera lens are either:
1) Clearly labeled as such IN ADDITION to the actual focal length
2) Point-and-shoot cameras with an attached lens that will NEVER be mounted to another camera.
09-12-2007, 06:45 PM   #12
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Fred500, focal lengths, crop factors, etc, can be rather confusing at times until you get the hang of it.

Here is a fairly good link with visuals that should help you better understand what everyone is saying.

Crop Factor
09-12-2007, 07:12 PM   #13
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I've never been comfortable with "Digital Lens" as often used on this Pentax Forum. As Entropy points out, the Pentax Digital Lenses are APS size but other companies have sensors larger and smaller. My friend's Canon 5D has a 35mm sensor so it's "Digital Lenses" illuminate a similar area as a film lens. They can be called "Digital" for reasons Enthropy points out. Pentaxians occasionally imply "Digital" is always APS sensor.

I have the two kit lenses and appreciate how small and light they are since it takes less glass when Pentax can assume APS sensors.
09-12-2007, 08:01 PM   #14
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QuoteOriginally posted by fredd500 Quote
Thanks for all your replies. I think maybe I am confusing myself. Is it right then that I cannot compare a film lense and a digital lense on the same camera? The field of view that people talk about would only become apparent if I took the two photos on the different media, i.e. film and sensor? Its not directly the affect the lens
It's exactly wrong that you can't compare film and digital lenses on the same camera. So wrong that it's perpendicular to correct. You *can* compare them on the same camera, and they'll look approximately the same.

A 28mm designed-for-film lens will have the same field of view as a 28mm designed-for-digital lens. (There will probably be other differences -- the for-digital lens may be lighter and smaller, and will have different optical coatings.) But if you put that designed-for-film lens back on your film camera body and take a picture, you'll notice that it's much wider. Specifically, what was 46 of the scene in front of you on the digital body is now 65 on film. (Although typically prints are cropped from that by a few percent.)


QuoteQuote:
has on the image being projected through it, but what effect the camera has on the image as it is captured - i.e. the digital sensor would capture less (because it is smaller) and therefore when the resulting image is enlarged to the same size, the subject would appear closer?
Yes, this is exactly right.

QuoteQuote:
What really started it off was the comment by my lecturer about what was a 'standard' lens, i.e. 50mm on a 35mm camera was considered standard, but on a digital one, 35mm was nearer to standard. When I looked through the VF, it seemed to me that 50mm was nearer the mark on my camera, the lecturer took a look, agreed and thought it was a bit strange.
Nearer to what mark?

It's actually a kind of arbitrary designation, but generally "officially" considered to be the focal length equal to the diagonal of your film or sensor -- which is why Pentax made their 43mm lens, which they tout as "true normal" for 35mm film. The equivalent on 1.5 crop-factor dSLRs is around 28mm. (Despite Pentax's one lens, it is indeed standard to round up to 50mm and 35mm respectively.)

As I understand it, it just happens that at around this mid-range focal length, neither wide-angle distortion or telephoto perspective compression are very apparent at normal print viewing size. There's nothing really magical.

QuoteQuote:
I guess I am most concerned that what I see through the vf is pretty much what I am going to be capturing on the senser.
Yes, that part is true. However, the only dSLRs I'm aware of with 100% frame coverage in the viewfinder are Nikon's just-announced D300 and D3. With everything else, a small percentage of the edge of the frame will be captured even though you didn't see it in the viewfinder. (This was common on film cameras too, and matched the practice of not giving you those edges on your prints either.)
09-13-2007, 03:59 AM   #15
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QuoteOriginally posted by mattdm Quote
Nearer to what mark?

It's actually a kind of arbitrary designation, but generally "officially" considered to be the focal length equal to the diagonal of your film or sensor -- which is why Pentax made their 43mm lens, which they tout as "true normal" for 35mm film. The equivalent on 1.5 crop-factor dSLRs is around 28mm. (Despite Pentax's one lens, it is indeed standard to round up to 50mm and 35mm respectively.)
When I look through my view finder, I see things the same size with the lens set to about 50mm as I do when I look with my naked eye. If I set the lens to 35mm, the image in the view finder is definately wider (i.e. the subject looks further away).

Or am I misunderstanding what I should be seeing? Is it all about the angle of what I see rather than the distance to the subject? With my naked eye, I see more on the periphery of my vision than I do looking through the lens when the lens is set to 50mm.

Sorry if I am being stupid, I am just trying to understand how all this optical stuff works and we all have to start somewhere...
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