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03-11-2012, 01:32 AM   #1
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Focal length and sensor sizes.

Hi,

I am a newbie in the field of DSLRs but have many happy years of Pentax film cameras behind me, so I am well aware of the basics. However there is one point I would like to get clearer in my head.

Back in the 35mm film days a “normal” focal length lens was 50mm – “normal” meaning that it gave the same perspective as the human eye (if I have understood that correctly). So a lens shorter than 50mm is considered to be wide angle, and one longer than 50mm, telephoto.


If I now take a 50mm lens from my old 35mm camera and put it on my K-x it behaves like a 75mm lens would have done on my 35mm film camera.

First question – is this because the sensor on my K-x is smaller than 35mm film?

Second question - this would suggest that a “normal” DA type lens for my K-x is still 50mm. Is this the case?

Final question – does this mean that 50mm is “normal” for all DSLRs (Pentax, Nikon, Canon etc.) as it was for all 35mm film cameras?

Thanks to anyone who can help put me out of my misery.


Last edited by Farmer_Terry; 03-11-2012 at 01:35 AM. Reason: clarification of question.
03-11-2012, 01:44 AM   #2
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1. Yes, it's a 24x16 sensor (about half the size)
2. No. A normal lens would be a 50mm / 1.5, so about a 35mm
3. FOV varies by sensor size as in this chart: https://www.pentaxforums.com/forums/pentax-lens-articles/93714-field-view-tab...d-645-6x7.html

https://www.pentaxforums.com/forums/pentax-lens-articles/90477-crop-factor-fo...ield-view.html

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03-11-2012, 01:59 AM   #3
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QuoteOriginally posted by Adam Quote
2. No. A normal lens would be a 50mm / 1.5, so about a 35mm
Thanks Adam - so just to clarify - a Pentax-FA 50mm F1.4 would be regarded a slightly telephoto on my K-x?

QuoteOriginally posted by Adam Quote
3. FOV varies by sensor size as in this chart: Field of View Tables, APS-C, 24x36, 645D, 645, 6x7
I wish I had managed to find this and this - it does answer most of what I was asking. I just didn't know "crop factor" was what I needed to look for.

Thanks.

Last edited by Farmer_Terry; 03-11-2012 at 02:10 AM.
03-11-2012, 02:15 AM   #4
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It's a bit hard to explain because 135 film camera's are sort of the exception.

I'll copy this part from wiki.
A lens with a focal length about equal to the diagonal size of the film or sensor format is known as a normal lens; its angle of view is similar to the angle subtended by a large-enough print viewed at a typical viewing distance equal to the print diagonal; this angle of view is about 53° diagonally.

This mean that the actual normal lens for the 135 format is 43mm, that's the reason why pentax brought out the FA43.
50mm comes from the early days, it was hard and expensive to make a good normal lens for the 135 format so they decided to settle for 50mm and that's why we see 50mm as standard today but it's actually a bit too long. 135 format is the only one with the skewed format, the larger formats all have true normal lenses.
True normal lens for aps-c is there for 38 to 30mm.


ps. angle of view is the deciding factor here not the focal length.
50mm will always be 50mm even if you have it in your hand for example, it's the nature of the lens.
However because of the different film/sensor sizes that what is captured is different, you capture more with a larg film then with a small film and thats where the difference comes from.


Last edited by Anvh; 03-11-2012 at 02:20 AM.
03-11-2012, 02:25 AM   #5
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Think you had a typo. Should be

True normal lens for aps-c is therefore 28 to 30mm.

Which is why there is a hankering for old 28mm lens even if they are manual.
03-11-2012, 02:38 AM   #6
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Sorry ANVH you posted at the sime time as me - I will look through your article. The following is in reply to Adam.


OK. I have read both of those articles, I understand what they are saying, and they do indeed answer some of my confusion.

However, the treatment there is purely arithmetic - i.e. to capture the same picture on an APS-C sensor and on 35mm film you need lenses with focal lengths in the ratio 1:1.52 because the sensor sizes are in the ratio 1:1.52.

But what about perspective? Wide angle lenses tend to exaggerate perspective and long lenses tend to compress it. (Don't they?)


If I put the same 50mm lens in front of 35mm film and an APS-C sensor surely the perspective in the both resulting images remains the same, even though mathematically the lens would behave as a telephoto (i.e. smaller angle of view) on the APC-C camera.

I think I may still be a bit confused here.
03-11-2012, 03:14 AM   #7
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Good explanations above. I'll give my own spin on it:

QuoteOriginally posted by Farmer_Terry Quote
If I now take a 50mm lens from my old 35mm camera and put it on my K-x it behaves like a 75mm lens would have done on my 35mm film camera.
A 50mm on 135/HF (half-frame) or APS-C does NOT behave just like a 75mm on 135/FF. The smaller frame crops the FOV to that extent, and DOF is different, and perspective will change because we'll shoot from a different distance to get the same framing of a subject. We can say that looking through the viewfinder, a 50/2 lens on APS-C would be similar to a 75/2.8 lens on 135/FF.

QuoteQuote:
First question – is this because the sensor on my K-x is smaller than 35mm film?
A smaller frame sees less of a projected image than does a larger frame. Try this: Cut a picture from a magazine. Draw a 60x45mm rectangle on it. Draw a 36x24mm rectangle inside that. Draw a 24x18mm rectangle inside that. Those are the frame sizes of MF, FF and HF cameras. The picture remains the same, but each frame crops a different amount. The amount of cropping, compared to 135/FF, is the crap.factor. (I prefer the German term format-faktor.

QuoteQuote:
Second question - this would suggest that a “normal” DA type lens for my K-x is still 50mm. Is this the case?

Final question – does this mean that 50mm is “normal” for all DSLRs (Pentax, Nikon, Canon etc.) as it was for all 35mm film cameras?
As mentioned, an optically 'normal' focal length is the diagonal of the frame, which for 135/FF is 43mm, which is shorter than the depth of the Pentax mirrorbox (45.46mm). Rangefinders often had lenses in the 35-40-45mm range because their rear elements could sit close to the film plane without resorting to exotic designs. SLR lenses had to accommodate the swinging mirror, so kit lenses were often 58mm, then 55mm, then 50mm, which were easily designed for speed. We see many more f/1.2s in those focal lengths than elsewhere.

We've discussed elsewhere the varied meanings of 'normal', derived from the Latin norma or carpenter's square. Normal can be: at right angles; within so many standard deviations of a mean; the diagonal of a frame; what is acceptable; what is familiar. 50-55-58mm were 'normal' due to familiarity, not optics; 45mm is closest to the FOV of one human eye, about 50 degrees. (Our binocular vision is 160 degrees, about 4mm on 135/FF.) But with an Olympus XA, 35mm was 'normal' (familiar), and so was 58mm for a Zenit. That's a pretty wide range of normality!

Optical normality varies with frame size (film or digital). For nominal APS-C it's 30.1mm. For my K20D it's 28.1mm. For Canon APS-H sensors it's 34.4mm. I don't know the precise size of a Kr sensor; check your manual. You can figure that a lens in the 28-29-30-31mm range will probably be 'normal'. But if you want to replicate the look of the old 135/FF kit lenses, then 35-37mm are on target. My Mir-1 37/2.8 would be closely FOV+DOF-equivalent to 55/4.

So if you want undistorted 'normal' images from your Kr, use a 28mm lens (also called "short normal"). Or a 31Ltd if you can afford it! For a view similar to your old 50mm on 135/FF, use a 35mm lens (also called "long normal"). Just remember the DOF and perspective differences. Better yet, just learn what each lens does on a specific camera. When I was shooting 135/HF and 135/FF and 6x6/MF and 9x12/LF on the same day, I didn't think about crap.factors, just about which lenses did what I wanted with each format.

Hope this helps!
03-11-2012, 03:59 AM   #8
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QuoteOriginally posted by Farmer_Terry Quote
OK. I have read both of those articles, I understand what they are saying, and they do indeed answer some of my confusion.

However, the treatment there is purely arithmetic - i.e. to capture the same picture on an APS-C sensor and on 35mm film you need lenses with focal lengths in the ratio 1:1.52 because the sensor sizes are in the ratio 1:1.52.

But what about perspective? Wide angle lenses tend to exaggerate perspective and long lenses tend to compress it. (Don't they?)


If I put the same 50mm lens in front of 35mm film and an APS-C sensor surely the perspective in the both resulting images remains the same, even though mathematically the lens would behave as a telephoto (i.e. smaller angle of view) on the APC-C camera.

I think I may still be a bit confused here.
First the conversion.
You're quite correct with what you say but to make this very clear.
50mm will always stay 50mm don't let that confuse you, all that the sensor size influence is that angle of view (and the circle of confusion).
So you won't get precisely the same photo if you use 35mm on APS-C and 50mm lens on FF but you do get so good as the same framing/view but for example the aperture is different because the focal length is different.
You get that part?


Now perspective
Perspective is not a lens thing it's about where you stand and point the camera.
Things get distorted because you stand close or far away and not because you use 12mm lens or a 200mm one, however you do often stand closer with the 12mm lens then you do with the 200mm one and this is where the misconception kicks in.
So 35mm on aps-c is used in the same way as 50mm on FF because the framing is the same and so is the distance from your subject, meaning that they have the same distortion.

03-11-2012, 05:10 AM   #9
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QuoteOriginally posted by Farmer_Terry Quote
Hi,

I am a newbie in the field of DSLRs but have many happy years of Pentax film cameras behind me, so I am well aware of the basics. However there is one point I would like to get clearer in my head.

Back in the 35mm film days a “normal” focal length lens was 50mm – “normal” meaning that it gave the same perspective as the human eye (if I have understood that correctly). So a lens shorter than 50mm is considered to be wide angle, and one longer than 50mm, telephoto.


If I now take a 50mm lens from my old 35mm camera and put it on my K-x it behaves like a 75mm lens would have done on my 35mm film camera.

First question – is this because the sensor on my K-x is smaller than 35mm film?

Second question - this would suggest that a “normal” DA type lens for my K-x is still 50mm. Is this the case?

Final question – does this mean that 50mm is “normal” for all DSLRs (Pentax, Nikon, Canon etc.) as it was for all 35mm film cameras?

Thanks to anyone who can help put me out of my misery.
You are correct in that the 50mm became the standard lens because it gives a simlar angle of veiw and perspective as the human eye.
That only works in 35mm though. It also works the other way around. If you use say a medium format camera, lets say a 645 the film is 2.7 times larger. and a 75mm lens is considered as standard because it aproximates 50mm on 35mm angle of veiw. With four thirds, which is even smaller than APSC then it would be a 25mm.
APSC has not really been established as all other formats, so its more a case of anything between say a 28 - 35mm could be regarded as standard.
03-11-2012, 08:26 AM   #10
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OK. Thanks for all this input, which I am finding extremely useful.

I do understand that a 50mm lens is a 50mm lens is a 50mm lens. The 50mm focal length is a characteristic of the pieces of glass involved and is in no way affected by any piece of equipment to which it may be attached. Basic physics.

Also I have been using the word "normal" in quotes as I also understand that normal means many things. I was using it to describe a lens that leads to images with the same perspective as the human eye - i.e. a lens that tends neither to the wide angle nor to the tele. I was therefore, fascinated by the definition anvh quoted from wiki of a "normal" lens in terms of the diagonal of the resulting image, and the viewing angle of a subsequent large print - something I have never heard in some 40 years of photography. I am still rereading that and thinking about it.
QuoteQuote:
Perspective is not a lens thing it's about where you stand and point the camera.
I am not at all sure about that one - perspective was something that artists came to understand long before the camera was invented. If I stand in the corner of a room with a wide angle lens, the resulting image distorts the room making it feel larger than it will look to my eye. An image taken from the same point with a "normal" lens will lead to no surprises when the viewer finally see the room itself.

Similarly, a photo of someone on a park bench taken with a long lens will mislead the viewer about the distance between the bench and a building in the background.
03-11-2012, 08:52 AM   #11
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QuoteOriginally posted by Farmer_Terry Quote
I am not at all sure about that one - perspective was something that artists came to understand long before the camera was invented. If I stand in the corner of a room with a wide angle lens, the resulting image distorts the room making it feel larger than it will look to my eye. An image taken from the same point with a "normal" lens will lead to no surprises when the viewer finally see the room itself.

Similarly, a photo of someone on a park bench taken with a long lens will mislead the viewer about the distance between the bench and a building in the background.
Yes and no, you're mixing things up here.

Here is a good read, that should help.
Distortion

QuoteQuote:
The perspective of a photograph is determined by the viewpoint that the photographer takes in relation to the subject.
...
Perspective is not affected by the lens focal length: it depends on the viewpoint only.

...

In contrast to optical distortion, perspective and geometric distortions are no lens aberrations. The apparent anomaly is emphasized by a wrong viewpoint for the image. Ideally each photograph should be viewed from a viewpoint that corresponds to the viewpoint of the photographer in relation to the captured scene. A telephoto image should be viewed from far and a wideangle photograph from close. A photograph with converging verticals looks more natural from a low viewpoint, and, similarly, egg faces improve when the photograph is viewed from close. Unfortunately the human eye does not cover a sufficiently large angle to view the entire image of a wideangle lens from nearby, so in practice the viewer takes a distance, notices the deformation, and speaks of distortion.

Last edited by Anvh; 03-11-2012 at 08:57 AM.
03-11-2012, 09:53 AM   #12
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QuoteQuote:
A lens with a focal length about equal to the diagonal size of the film or sensor format is known as a normal lens; its angle of view is similar to the angle subtended by a large-enough print viewed at a typical viewing distance equal to the print diagonal; this angle of view is about 53° diagonally.
QuoteQuote:
The perspective of a photograph is determined by the viewpoint that the photographer takes in relation to the subject.
Well. Have I learned a lot today !

Thank you all for your time and patience - that is a thoroughly enlightening discussion even if I don't entirely understand it all yet.

I will spend some time on the suggested articles and see how I get on.

Thank you once again.
03-11-2012, 10:57 AM   #13
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The text is a bit hard though since it's quite technical.

You can try the perspective thing out at home, often if you see it for yourself you believe it for sure.
Take a zoom lens and take a photo off something while you stand at the same spot.
So if you have the 18-55 kit lens, take a photo at 18mm and 55mm standing at the same place, now set the lens at 18mm and move close to the subject so it fills the frame the same as the 55mm photo.

You will see that the 18mm photo and 55mm has the same perspective, to prove that, crop the 18mm photo so that you get the same photo as the 55mm one and you will see that they match.
Now with the second 18mm photo where you moved you see that it doesn't match the 55mm photo at all in terms of distortion.


edit:
the prime vs zoom lens example shows the differnce perfectly.
http://www.cambridgeincolour.com/tutorials/camera-lenses.htm
03-11-2012, 12:07 PM   #14
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QuoteOriginally posted by Farmer_Terry Quote
I am not at all sure about that one - perspective was something that artists came to understand long before the camera was invented.

There is a theory that it was the invention of the camera (camera obscura) that allowed artists to learn the rules for perspective drawing.

DAZ
03-12-2012, 02:44 AM   #15
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Thanks again Anvh - I will certainly try that experiment.
QuoteOriginally posted by DAZ Quote
There is a theory that it was the invention of the camera (camera obscura) that allowed artists to learn the rules for perspective drawing.
That is interesting, and sounds quite likely.

What a useful resource this site is.
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