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01-30-2012, 03:47 PM   #1
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i can't believe i am asking this after so many years

so i have always assumed i understand focal length conversion between 35 and aps
but today i realized that my m50 1.7 gives the same fov as my 18-55mm set on 50.
shouldnt my 50 look like a 75mm?

then i remembered 50mm was the old natural fov lens so maybe this means that the 18-55 printed on my da lens is actually the digital equivalent?

basically, i want to grab either a 77 or 43 ltd depending on how they'd look on my K5, but i was initially worried that the 77 would give me the same 100 view as when i set my da 50-200 to 100... so can someone explain this to me once and for all? the part about why my DA lenses are the same as my m/fa lenses even though there is supposed to be a focal length multiplier.

01-30-2012, 03:52 PM   #2
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With a few exceptions that are usually noted somewhere a focal length is a focal length. The difference between the m50 and the kit zoom is the size of the image circle they project- the m50 covers a 35mm film frame, the kit lens coves an APS-C sensor and would vignette on a 35mm film frame.

Other wise the FOV is the same between the lenses.
01-30-2012, 03:56 PM   #3
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QuoteOriginally posted by dominikkolendo Quote
so i have always assumed i understand focal length conversion between 35 and aps
but today i realized that my m50 1.7 gives the same fov as my 18-55mm set on 50.
shouldnt my 50 look like a 75mm?

then i remembered 50mm was the old natural fov lens so maybe this means that the 18-55 printed on my da lens is actually the digital equivalent?

basically, i want to grab either a 77 or 43 ltd depending on how they'd look on my K5, but i was initially worried that the 77 would give me the same 100 view as when i set my da 50-200 to 100... so can someone explain this to me once and for all? the part about why my DA lenses are the same as my m/fa lenses even though there is supposed to be a focal length multiplier.
The field of view is dependant on the focal length and the sensor size, not on whether it's a DA or FA lens. So the FA35 gives the same FOV on a Pentax DSLR as the DA35. Same focal length, same sensor.
01-30-2012, 03:57 PM   #4
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thank youso much for clarifying that so fast. i jsut started scratching my head to day becasue i never actually thought twice about the numbers. with that info i'm finally ordering my first "nice" lens, the 77... so glad.

01-30-2012, 03:58 PM   #5
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QuoteOriginally posted by dominikkolendo Quote
so i have always assumed i understand focal length conversion between 35 and aps
but today i realized that my m50 1.7 gives the same fov as my 18-55mm set on 50.
shouldnt my 50 look like a 75mm?

then i remembered 50mm was the old natural fov lens so maybe this means that the 18-55 printed on my da lens is actually the digital equivalent?

basically, i want to grab either a 77 or 43 ltd depending on how they'd look on my K5, but i was initially worried that the 77 would give me the same 100 view as when i set my da 50-200 to 100... so can someone explain this to me once and for all? the part about why my DA lenses are the same as my m/fa lenses even though there is supposed to be a focal length multiplier.
Just forget you ever heard the term crap.factor.

A 50mm lens is a 50mm lens whether it is designed for 135 or designed for APS-C. The m50 simply has a larger image circle than a 50 designed for APS-C.
01-30-2012, 04:17 PM - 1 Like   #6
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You're confusing FOV and apparent magnification.

The M lens throws the same size image on the sensor and the film. It is just that the sensor is physically smaller. Take a print from a film shot, cut away the outer one-third of it, and that's what your APS-C photo will be. You lose some around both sides, the top, and the bottom, but what is left in the middle is identical, as you have noted. Naturally, if you take this reduced image and then blow it up to be the same physical size as the other one, it is going to look like it has been magnified 1.5x and that is the "crop factor" people talk about and why it is said a 50mm on digital is "equal" to 75mm on film. The actual focal length, though, doesn't change.

The effect is most noticeable on short focal length lenses and negligible on longer ones. Think of it in terms of degrees covered in the field of view rather than as focal length and you will easily understand why this is:

You lose 1/3 of the field of view, so a lens that captures 180 degrees on film would capture 120 degrees on APS-C......a huge difference. A lens that captures 6 degrees on film would capture 4 degrees on APS-C.

So a fisheye would lose 30 degrees off each side (and top/bottom), which is very noticeable and would fool your eyes into thinking the image magnified by quite a large bit. The other would be a long telephoto and you'd only lose 1 degree off each side and the apparent magnification would not appear anywhere near as severe as it does with the short lenses.

The crop factor refers to an apparent magnification, not an actual magnification.

Lenses designed specifically for digital are different mainly in that they are designed to have the light rays traveling in more of a straight line out the rear of the lens and onto the sensor. Film really doesn't care if light rays hit it at an angle, but apparently digital sensors work best with light hitting the surface as perpendicular as possible. Never a problem in the exact middle, of course, but the farther you go toward the edges the more of an angle the light is traveling at....unless you monkey with the lens design and try to make it come out perpendicular to the sensor.

Last edited by Mike Cash; 01-30-2012 at 04:23 PM.
01-30-2012, 04:33 PM   #7
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daggommit mike, as soon as i thought i understood this you confuse me.
forget film for a second. i will never in a million years touch it. dont care about the math anymore and i dont care to understand this its jsut drivign me crazy.

maybe if i just get a yes or no to this question i will be satisfied, and i only ask becasue of "The effect is most noticeable on short focal length lenses and negligible on longer ones. Think of it in terms of degrees covered in the field of view rather than as focal length and you will easily understand why this is:"

did you mean that as between 35mm and aps-c ? casue i dotn care about 35mm vs aps-c, i only want to know how lenses designed for fiil will capture an image compared to what i consider 18mm (teh 18mm settign on my da 18-55)

ok so my question.
if i buy say... a FA24mm and pop it on my k5, would it give me the same image as if i had my 18-55 set on "24" ? (sharpness etc notwithstanding)
becasue thats what i thought i understood with all the previous answers
01-30-2012, 04:36 PM   #8
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QuoteOriginally posted by dominikkolendo Quote
ok so my question.
if i buy say... a FA24mm and pop it on my k5, would it give me the same image as if i had my 18-55 set on "24" ? (sharpness etc notwithstanding)
becasue thats what i thought i understood with all the previous answers
Yes it will.

01-30-2012, 04:36 PM   #9
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QuoteOriginally posted by dominikkolendo Quote
ok so my question.
if i buy say... a FA24mm and pop it on my k5, would it give me the same image as if i had my 18-55 set on "24" ? (sharpness etc notwithstanding)
Yes.

Field of view, 'apparent magnification' would be the same if you shot the 24 on the K5 and the 18-55 at 24 on the K5.

(You'd have more ability to control DOF with the 24, since it goes to f/2, but I don't think that's what you're asking.)


.
.

Last edited by jsherman999; 01-30-2012 at 09:53 PM.
01-30-2012, 04:39 PM   #10
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thank you everyone, seriously. that was better than tylenol for this headache.
01-30-2012, 05:57 PM   #11
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You're the one who brought up 35mm (film) and digital equivalent, so I really don't understand why I have to catch an earful of crap for then attempting to explain it in that context.

I don't know how anyone could explain crop factor without referencing film.

Does this boil down to your wanting to know if a 50mm prime lens is going to take a picture that looks just like one take with a zoom lens set at 50mm? That's what you seem to be asking about here. The answer is "Yes....why would you expect otherwise?"

If you don't want crop factor explained, then don't ask about crop factor and dont bitch when you get an explanation of crop factor.
01-30-2012, 09:51 PM   #12
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A 50mm lens is a 50mm lens no matter what camera it sits on. The frame (film or digital) sees a certain amount of the projected image; different frames see different amounts of the image. A smaller frame 'crops' the image more than a larger frame, that's all. The lens is just an innocent bystander.

Back in the day, I had an Olympus Pen-FT half-frame 35mm SLR. As a half-frame (135/FF) its frame was very nearly the size of an APS-C sensor, so the same crap.factor applies. I bought a 400mm lens an thought OH BOY IT'S LIKE A 600MM LENS! But it wasn't. It was just a 400mm image with the sides chopped off.

That 400mm tele was a Spiratone in T2 mount. I have one just like it now. I can mount it on a K1000 (full-frame) or a K20D (half-frame). With other T2 adapters it could be mounted on an m4/3 or 645D or even a Q, very different frame sizes -- but it remains a 400mm lens.
01-31-2012, 05:03 AM   #13
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The lens always stays the same, the focal length always stays the same, whatever camera it is mounted on. The only difference is what portion of the image circle the sensor actually sees.

For many years, camera companies sold cropped sensors as a way of increasing focal length, which as RioRico says, is not true. You just see a narrower angle of view with a cropped sensor than a full frame sensor (film).
01-31-2012, 05:23 AM - 4 Likes   #14
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QuoteOriginally posted by Mike Cash Quote
You're the one who brought up 35mm (film) and digital equivalent, so I really don't understand why I have to catch an earful of crap for then attempting to explain it in that context.
I don't know how anyone could explain crop factor without referencing film.
Does this boil down to your wanting to know if a 50mm prime lens is going to take a picture that looks just like one take with a zoom lens set at 50mm? That's what you seem to be asking about here. The answer is "Yes....why would you expect otherwise?"
If you don't want crop factor explained, then don't ask about crop factor and dont bitch when you get an explanation of crop factor.
To be fair Mike I don't think anyone has given you an ear full of crap and your attitude kinda sucks... I'm hoping you just had a bad day...
This is a beginners forum so taking your bat and ball home and having a cyber-tantrum is: a) fairly unhelpful and: b) makes you look like a pr**k...
Why bother being plain argumentative?!

Normally your advice is sound... You are a credit to this forum...
In this instance however I reckon you let yourself down...

Sorry, but I felt it had to be said (out loud.)...
01-31-2012, 08:05 PM   #15
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QuoteOriginally posted by DaveHolmes Quote
To be fair Mike I don't think anyone has given you an ear full of crap and your attitude kinda sucks... I'm hoping you just had a bad day...
This is a beginners forum so taking your bat and ball home and having a cyber-tantrum is: a) fairly unhelpful and: b) makes you look like a pr**k...
Why bother being plain argumentative?!

Normally your advice is sound... You are a credit to this forum...
In this instance however I reckon you let yourself down...

Sorry, but I felt it had to be said (out loud.)...
He asked for an explanation of the difference between the use of lenses on film and on digital.

I diligently wrote up a explanation of the difference between the use of lenses between film and digital,

Then I get told off for daring to mention film in my explanation.

Which makes me an argumentative prick throwing a childish temper tantrum. (I reckon you let your own self down with such gratuitous ad hominem hyperbole, so long as we're sharing our reckonings).

It is like somebody asking what the difference between Judaism and Christianity is and then going, "Waaaa!!!! Don't mention Jesus! Don't care nothing about him. Don't want to hear it."

It is ungracious to ask for an explanation of something and then carp about the nature of the explanation, especially when the explanation covers exactly is asked about. If pointing that out makes me the prick, then I suppose I'll just have to be the prick. I don't appreciate being taken to task for doing precisely what I was asked to do just because the person asking did a piss-poor job of making it clear what it was he wanted.

I didn't clue in to what he was asking at first because of a couple of factors:

1. Apparently he already had from personal observation the answer to what his real question was.

2. The way he phrased it made it look like he wanted to know about crop factor.
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