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Old Sep 1, 2005, 7:55 PM   #11
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Thanks for all of your help fellas. I think I will buzz on over to Samy's and check out the 28mm 1.8, the 50mm 1.8, the 30mm 1.4 (if they have it in stock) and the 24mm1.4L. I'm guessing the 24 is about $1200. Yikes, but hey I'm getting on in years and maybe deserve a treat for myself. You can't take it with you.

I wonder if the folks at Canon have anything in the pipeline at 31mm and 1.4?
I'll bet that Sigma sells a boatload of their 30mm, especially if reviews are favorable. I'm guessing that I will end up purchasing either the 24 or the 30.

Again thanks for your responses. This board can be a real resource for newbies.

yussi
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Old Sep 2, 2005, 12:56 PM   #12
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Best of luck with your quest for the ideal prime lens Yussi - I and the others here wish you luck, and would be interested in hearing how it works out photographically in the end.

But I think it could be irresponsible of me to not mention this opinion: That you would be better served with a zoom lens. This does not invalidate the quest for the ideal prime, which I certainly appreciate, but IMO you could get more usability with a zoom lens.

Here are some good candidates, among many:

Tamron 17-35 f/2.8-4

Canon 17-40 f/4.0 L

Canon 17-85 f/4-5.6 IS

I have personal experience with only the 17-40 L, which I find satisfactory (which is saying something , but have good faith on the others based on reviews.

I find that I do the majority of my shooting with the 17-40, except for distant animals, and very often on 17. I also report that I found my 17-40 to match / outperform a 28 f/2.8 prime. That'sthe basis of this post.

Primes can be excellent, using a few good ones as a rule, and also, rarely, as with Henri Cartier-Bresson (HCB) sometimes the majority of work can be done with a single focal length. As a matter of speculation, I mention that the match between camera viewfinder and unaided eye might be why some like HCB liked the 50 (and with a 20D (0.9x viewfinder) at least, the 50 primes match the eye quite well).

Note also that DoF is determined by distance and focal length, not crop factor, so a 28 with have the DoF of a 28, not of a 28 * 1.6.

I hope I haven't confused / distracted/ offended / bored you; I wanted to post my honest opinion, thata zoom would probably serve better; although I do agree that primes can do a lot, and even the 50 cropped.
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Old Sep 2, 2005, 5:37 PM   #13
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My line of thinking goes something like, "If you're going to buy a prime, make sure you abuse its advantage over zooms, which is aperture"

Some can say primes are optically better, but the bests of zooms will hold their ground.

But (for instance) the sigma 30mm f/1.4 can let in 4x as much light as any of the fastest zooms in existence. That's a major advantage! So it's either that or the convenience of multiple focal lengths. The ONLY time I would consider a prime with an aperture of 2.8 or smaller would be if I was getting a macro lens.
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Old Sep 2, 2005, 6:32 PM   #14
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BoYFrMSpC wrote:
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The ONLY time I would consider a prime with an aperture of 2.8 or smaller would be if I was getting a macro lens.
i wouldn't mind a 300 or 400mm 2.8!!
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Old Sep 3, 2005, 12:05 AM   #15
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Mad--I do have the 17-85 and it has served me well. I am sorry if I misled you by not giving enough info. I am interested in a fast prime because of the good memories that the F-1 50mm 1.4 combo gave me. There is something to be said for the very shallow dof of that aperature, not to mention the low light capabilities.

BoY--I agree. If you are going to get a prime you should max out the advantage.

peri--You have a nice collection of shots there, done with a variety of lenses. That gives me something to shoot for.

I do have a few days off next week so I should be able to check out all of the options given to me here. Thanks again.

yussi
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Old Sep 3, 2005, 6:05 AM   #16
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Madwand wrote:
Quote:
Note also that DoF is determined by distance and focal length, not crop factor, so a 28 with have the DoF of a 28, not of a 28 * 1.6.
As written that's very misleading - the situation is more rather complex than that.

[Example 1 below shows where you are wrong to think of it that way, and example 2 shows where you are right.]

Allow me to ramble on for a bit: :-)

What is DOF? Quite simply it's that area of the printed photograph which appears to be acceptably in focus to the viewer. But that sounds rather subjective, and in fact it is, but there are a set of standards which are common in the optical world for a "normal" viewer under "normal" conditions these give us a figure for Circle of Confusion and the Leica standard CoC for 35mm film format is 0.025 mm - this is the most commonly used standard.

DOF is determined by 4 variables:
1. Focal length
2. Focal distance
3. Aperture
4. Circle-of-Confusion

http://www.dofmaster.com/equations.html

Now CoC is generally calculated for 2 variables that are assumed to remain constant in DOF calculations to get that Leica figure of 0.025mm.
1. Print Size
2. Viewing Distance.


Of course cropping a photo doesn't change the DOF, but cropping and then enlarging to keep print size constant (which is what a 1.6 crop camera does) most certainly does change DOF!

Why? Simply because detail which previously appeared as points (and therefore in focus) now appears as circles (and therefore out of focus).

Similarly taking 3 steps backward changes the DOF. Billboards look fuzzy close up, but sharp when driving by at a distance.

Note that this has nothing at all to do with digital and is purely based on "format".

Example 1:

100mm lens, 30m focal distance, aperture f4:

Format 6x7cm film -> DOF = 88.8m

Format 35mm film -> DOF = 24.7m

So clearly as you crop (& enlarge to keep print size constant) your DOF is DECREASING!

If you want to play with the calculations yourself :

http://www.dofmaster.com/dofjs.html

But wait I hear you cry! DOF on a digital camera is GREATER than on a FF. How could that be?

Well, it's very simple really, what people are referring to there is the fact that when you make an adjustment for FOV you are changing the focal length of the lens. And the SQUARE of the focal length and the difference between focal distance and hyperfocal distance is used in the DOF equation so the change in focal length more than compensates (sometimes quite dramatically so - see next example) for the reduced DOF from the change in format.

Example 2:

80mm lens, 30m focal distance, aperture f4:

Format EOS1DmkII -> DOF = 49.1m

Format Canon 20D -> DOF = 24.4m

But let's change the
Canon 20D to use a 50mm lens to give the same FOV:

50mm lens, 30m focal distance, aperture f4:

Canon 20D -> DOF = 319.4m

That's a huge difference! Because the 30m distance is now close to the hyperfocal distance for those parameters.

Now consider that the 20D sensor is actually quite large compared to the tiny sensors on P&S cameras and you can see why it's very hard to get anything more than a couple of meters away OOF with the small cameras!

Now all of what has come till this point would apply equally to film cameras with different film sizes.

The source for all of these equations can be found at http://www.dofmaster.com

That's the basics, and it's worth reading the equations so that you understand the principles.

http://www.dofmaster.com/equations.html


But....

There is a problem with the DOF calculations at dofmaster, and that is that the CoC is worked out assuming constant pixel density between sensors, and is therefore essentially a function of the crop factor. While this is a good simplifying assumption and may have been more accurate in the early days of digital, things are a little more complicated now; the actual CoC can change not just because of the "crop & enlarge" element, but also because the density of the pixels is not uniform between cameras.

To get a good visual on this click the following link and scroll down to the colourful bit.

http://www.dpreview.com/articles/canoneos5d/

So how can one get a more accurate calculation for the various digital cameras?

You can download this spreadsheet and play with the values if you're still awake. :blah:

http://visual-vacations.com/photogra...or_JLW-DAK.xls










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Old Sep 3, 2005, 11:14 AM   #17
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peripatetic wrote:
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Madwand wrote:
Quote:
Note also that DoF is determined by distance and focal length, not crop factor, so a 28 with have the DoF of a 28, not of a 28 * 1.6.
As written that's very misleading - the situation is more rather complex than that.

[Example 1 below shows where you are wrong to think of it that way, and example 2 shows where you are right.]

Allow me to ramble on for a bit: :-)
It wasn't my intent to give a technical explanation of DoF here. Can you simply this for me and tell me where you think my statement was very misleading for the intended purpose of this thread? The context is this: Comparing a 50mm lens on a full frame camera with a 50/1.6 lens on a cropped DSLR, where the intent is to reduce the DoF?
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Old Sep 3, 2005, 12:52 PM   #18
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yussi wrote:
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Mad--I do have the 17-85 and it has served me well. I am sorry if I misled you by not giving enough info. I am interested in a fast prime because of the good memories that the F-1 50mm 1.4 combo gave me. There is something to be said for the very shallow dof of that aperature, not to mention the low light capabilities.
Phew! Best of luck again.
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Old Sep 3, 2005, 4:04 PM   #19
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You're correct of course that when using a crop camera it's as well to be aware of how DOF is affected.

Well let's see you claim that:

1. DOF is determined by focal length and focus distance, which is correct.
2. DOF is not determined by crop factor, which is not true and is therefore misleading.

Where might this be important in the context of the thread? It seemed to me that you were suggesting this was a negative factor, perhaps I misunderstood that.

It seems that you were suggesting that you wouldn't be able to effectively isolate the subject, and I would suggest that isn't really true - when you are shooting with a focus distance which is not close to the hyperfocal distance, then the DOF difference is not necessarily large enough to be an issue.

Well let's say we consider the an aperture of f1.4 at a likely indoor snapshot focus distance of say 2m.

On a FF camera with a 50mm lens the DOF would be 0.13m, on the XT with the 30mm it would be 0.24m - I guess it's up to the individual to say whether that's a good or bad thing.

One other thing to keep in mind is that on the XT and 20D the AF system is notoriously less accurate than the 1-series cameras and having that little extra DOF to play with means the camera will be less likely to let you down.

And of course the point could also be that on a snapshot lens it's actually a good thing to have greater DOF, and in the context of your post it seemed that you regarded it as a definite negative.

But I notice that you said "Note" not "Beware" so I may have the wrong end of the stick for which I apologise. And after all I must admit that your comment was not *very* misleading, only a little and so you deserve a second apology.

Sorry x2.
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Old Sep 3, 2005, 5:47 PM   #20
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Thanks for the numbers, they might help the OP. (I'm too lazy to check them myself

Although I didn't state (2), I did omit mentioning the print / image magnification issue (i.e. format issue), so I suppose that your elaboration is not unfair, and could help some people. I understood it to be less of a factor than the focal length, in general, which I think your numbers show.

Typically, I agree with more area in focus = better, but the OP here was looking for the full frame 50 1.4 DoF (i.e. very shallow). I think that the cumulative information here is better than the simple and wrong assumption that a 50/1.6 lens on a 1.6x crop factor is exactly the same as a 50 lens on a full frame with respect to DoF.

Cheers.
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