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TP: standard lens and the crop idea

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dCap View Drop Down
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Post Options Post Options   Quote dCap Quote  Post ReplyReply Direct Link To This Post Topic: TP: standard lens and the crop idea
    Posted: 12 June 2007 at 13:02
spilling out from the Line-up thread. And we often see a similar question about the DT or APS-C lenses. Is it a 18-70 or not? becuase its a digital only lens, is it an effective 18-70 on digital, or a film-like 18-70 ... it can be confusing. But the 18-70 is an 18-70 ... it will feel like a 27-105 though, if you are used to film lenses.

The focal length numbers are not going to change. It all depends on the size of your film diagonal or your chip diagonal to understand what that lens will feel/act like.

So, on film, the diagonal of the 36x24mm film area is actually about 43mm. So our standard lens way back in the olden days of film should have been a 43mm. There were some 45mm lenses about. But in the main it was a 50mm lens treated as the standard.

Our 5D/7D/a100 chip has a diagonal of 28mm ... this is where the famouse 1.5x crop comes from. 28 x 1.5 = 43 ... ish. The numbers are actually:
... film: 43.27mm
... 7D: 28.26
... the crop factor being 1.53x
Remember though that your 35/1.4 G is probably a 33mm lens or a 36mm lens. Odd numbers are marketing nightmares, so it;ll get rounded. That's why you shouldn't lose any sleep about applying a 1.53x crop, 1.5x about right. Very very very few people make an odd focal length. Mamiya had a 127mm, Pentax do a 77mm for example.

Now, I have an Olympus E-400 which has an even smaller chip than my a100. So the effective multipler for me is 2x for my Oly lenses.

I'll take a lens that is available in all three formats, the WONDERFUL Sigma 24/1.8 EX DG (HSM on 4/3)
- film, its a 24mm pretty wide lens
- on a100 its a 24mm, but feels like a 36mm, just a wide angle
- on the 4/3 system its still a 24mm lens, but feels like a 48mm, standard lens.

On your a100 its still a 24mm lens, but you are only able to view the middle chunk, thanks to the 1.5x crop. personally i think using this on a 4/3 isn't a great plan. That really is a 2x crop, the lens can't get any better the more you zooooom in on its image circle.

So, lens makers can do two things:
- make lenses to fit film cameras (1x crops), and give them a digital coating, so that they perform better on a digital camera ... even though (for the alpha mount) we are currently cropping part of that image circle. You see people in here trying to protect their film size arrangement in the hope that Sony will make a full frame digital.
- design lenses to be 1.5x crop fit only! e.g. Sony DT 18-70, Zeiss 16-80, Tam 17-50/2.8, Sig 30/1.4 is digital crop only. Often refered to as APS-C ... these by nature are smaller in size, they are only needing to make an image big enough for a smaller sensor. Advantage, size, wieght, cost.

Has anyone seen a lens for a medium format camera. Their standard 80mm lens is rather large compared to our old 50.

While I'm here, Olympus and the 4/3 have another option. We can take a FILM lens, like the Sig 24 and have it on 4/3 ... but all I'm doing there is carrying around a very large lens and only using half of what it can do. Or they can take the the APS-C lenses like the Sig 30/1.4 (already a small lens, made only for APS-C cameras), but I'm still cropping a part of that, its a 60mm to me, and it was the best on balance for me to get. With Olympus and the 4/3 system though, they have made lenses perposefully for the 4/3 chip (same idea as Sony and their DT 18-70). But where Oly differ is that they have made pro lenses in thier size, like the 35-100 f2 (its an effective 70-200mm f2 lens). Panasonic/Leica have just made a 25/1.4 for the 4/3 system - this would be perfect for me, it is made exactly for the 4/3 chip ... but its also L700 (US$1400), so I decided to get the much much much cheaper Sigma.

So ... if you really really want a standard lens on an alpha100 ... its a 28mm lens, the exact size of the chip diagonal. Might explain why Sony kept the 2.8/28 in the line-up? they should make us a 28/1.4 really.

There are probably better explanations about this on the net. But the mm don;t change. Look at the numbers quoted on your compacts, tiny tiny chip. Really small numbers, but feels like a 35-100 or something like that.
2005 = Dynax 7D and various prime lenses and Maxxum 7D
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Bob J View Drop Down
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Post Options Post Options   Quote Bob J Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 13:35
I always prefered a wider standard myself - my rangefinders were 38mm and my original 35mm SLR standard lens was a 40mm f1.8 'pancake' - I always thought the Minolta 45mm f2.0 should have been more popular..

Bob
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Post Options Post Options   Quote dCap Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 14:51
Yep, I used CAF 35/2 on an EOS film or Ultron 35/1.7 on Bessa

so I should be using a Min 24 (hence the Sig 24 I enjoyed so much) or a 17.5? in Oly? Going to do for 60/1.4 though
2005 = Dynax 7D and various prime lenses and Maxxum 7D
then I moved to Nikon, then I added some Olympus
30-Nov-2018 = Panasonic G80 + 12-35/2.8 <<< planning to shoot with just this camera/lens in 2019
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Post Options Post Options   Quote eldonito Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 15:28
So, talking normal lenses...

Oly: 21-25 mm
Alpha: 28-30 mm
Film: 42-50 mm
Medium: 80-85 mm

Lenses with such f. lengths would offer a similar, normal FOV in their respected bodies, but, for the same f/number + focus distance a different DOF, going from deep to shallow in the list above. Right? Maybe something extra to hurt your brain with :-)

Edited by eldonito - 12 June 2007 at 15:28
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Post Options Post Options   Quote Bob J Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 15:39
Originally posted by eldonito eldonito wrote:

So, talking normal lenses...

Oly: 21-25 mm
Alpha: 28-30 mm
Film: 42-50 mm
Medium: 80-85 mm



Well not quite - as dCap says the optimal should be 42 for 35mm rather than the bottom end, but is more usually 50 - in some cases even up to 58, so the range 35-58 is reasonable for 35mm: the bottom end of that is more normal on rangefinders, and the top end for SLRs (mainly because a 35 on SLR is usually mildly retrofocus due to the need to clear the mirror..)

'Normal' is a very fuzzy line :-)

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Post Options Post Options   Quote eldonito Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 16:05
Originally posted by Bob Janes Bob Janes wrote:


'Normal' is a very fuzzy line :-)

Bob


I agree with this, my figures weren't meant to be exact... just wanted to raise the point that a set of lenses that provide the same FOV in different formats will however differ in DOF.

Which is why my phone's 4.8 mm lens, which gives a normal perspective (so its sensor size gives a 10x crop factor? no idea...) never has to struggle focusing much! Just rambling really...

Edit: The reason for the whole kerfuffle being the implication for a possible Sony FF camera... If you used the following settings on a 1.5x camera: 30 mm, f/2.8, 1 m distance you would get a total DOF of 10.4 cm. Whereas if you had a FF camera and wanted to get the same perspective, you would have to use a theoretical 45 mm lens, which at f/2.8 and 1 m distance would produce a total DOF of 4.54 cm. All this according to the DOF calculator. Could make or brake a portrait :-)

Edited by eldonito - 12 June 2007 at 16:21
 



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Post Options Post Options   Quote gvknight Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 16:47
my head hurts
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Post Options Post Options   Quote Bob J Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 16:55
When we bring in DoF is when I start to lose it too:

My understanding was that DoF was linked to magnification and distance, so that the DoF from a 50mm macro is the same as with a 100mm macro framing the same subject - the wider focal length being offset by the closer distance...

...but then you get to p&s cameras with those tiny focal length lenses where pretty much everything is in focus, but if they are at the same magnification, shouldn't they have the same DoF..!!??

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Post Options Post Options   Quote Turerkan Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 19:46
bob janes: magnifaction is not related to the area of the object captured in the shot.

magnifaction is the enlargement ratio of the image falling on the sensor compared to the real life scene. therefore the area captured is a function of sensor size and magnifaction.

a P&S with a 1:1 lens would probably shoot living cells and alike:p
when giving the same FOV, p&s lens does much less magnifaction, hence the DOF observation doesnt contradict the science:)
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Post Options Post Options   Quote Bob J Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 19:57
You are quite right - and it is one of those things that, I can just about get my head around for about 10 mins, but then find myself confused about again. Must stop thinking about it before my brain explodes... :-)

Bob
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Post Options Post Options   Quote foot Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 19:58
so sigma makes a HSM 30/1.4 for the oly, but not the minolta/sony mount???

That "market share" explanation seems fishy to me...
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Post Options Post Options   Quote eldonito Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 20:04
Originally posted by Bob Janes Bob Janes wrote:

You are quite right - and it is one of those things that, I can just about get my head around for about 10 mins, but then find myself confused about again. Must stop thinking about it before my brain explodes... :-)

Bob



Heheh, I know the feeling exactly!
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Post Options Post Options   Quote CTYankee Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 20:12
Sigma has yet to make an HSM lens for the Maxxum/Alpha mount. Who knows whether it's legal/technical/economic ... I have to think that if they were to produce a line of HSM lenses they'd sell a lot given all the clamoring for more affordable SSM lenses !
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Post Options Post Options   Quote mhohner Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2007 at 20:27
Originally posted by dCap dCap wrote:

Often refered to as APS-C ... these by nature are smaller in size, they are only needing to make an image big enough for a smaller sensor. Advantage, size, wieght, cost.

In fact, this is a bit of a myth, or misconception, if you like calling it that. If you'd build two lenses with the same focal length, max. aperture, build quality, lens mount, etc., only one with full frame coverage and the other with APS-C coverage, you'd find that the difference between them is small. They'd be of about the same size, weight and cost.

What's really meant is the following: If you wanted to have a lens with a given angle of view and aperture, you'd need only a shorter lens for a smaller format camera, and that shorter lens would be smaller, lighter and cheaper than the longer lens for a larger format camera.

Just my 2.5 cents...
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