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Old Jun 10, 2009, 4:26 AM   #1
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Default Lenses for 1.X crop factor dSLRs

I asked this on dpreview, but I'm still confused.

I take alot of photos at 28mm (thats what my SX10 says is the 35mm equivalent). So with a dSLR (I'll be getting the D40 soon) I want to take the same sorts of photos i.e. I need a lens that does 28mm. Since the D40 has a crop factor of 1.5 this means that I actually want a lens that 'does' 17/18mm (IIUC). Now Sigma have specific 'DC' lenses. These are for the non-full size dSLRs (again IIUC).

So ... do I have to take into account the crop factor when buying a DC lens from Sigma? I'm thinking that the 17-70mm f/2.8 would be a really cool 'second' lens (fast, motor inside for AF, seems that the reviews aren't bad, price isn't too bad). But if I don't need to take into account the crop factor, then I shouldn't even be looking at the 17-70mm.

Edicate me please !!

Matt
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Old Jun 10, 2009, 4:58 AM   #2
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It doesn't make any difference if the lens is designed for a smaller APS-C size sensor or not. You'll still need to multiply the focal length of a lens by 1.5x to see what lens you'd need on a 35mm camera to get the same angle of view.

The focal length of both lens types is the same, as is the angle of view on an APS-C size sensor. But, with lenses designed for an APS-C size sensor, the image circle is smaller (so, you'd get vignetting if you try to use one on a camera with a larger sensor or film size without cropping).

The Sigma 17-70mm f/2.8-4.5 DC HSM Macro would give you roughly the same angle of view on a D40 that you'd have using a 27-105mm lens on a 35mm camera.

Note that with the D40, you'll need to make sure any lens you buy for it has a built in focus motor if you want Autofocus (that body does not have a built in focus motor, so lenses you use on it will need one if you want AF). Sigma made more than one version of their 17-70mm. So, make sure you get the newer HSM (Hypersonic Motor Focusing) version for Autofocus on a D40.
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Old Jun 10, 2009, 5:17 AM   #3
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I'm thinking that the 17-70mm f/2.8...
Note that the Sigma 17-70mm DC lenses you're looking at are f/2.8-4.5 (not f/2.8 throughout the zoom range). IOW, you only have f/2.8 available at the wide end of the zoom. If you want a lens that can provide f/2.8 throughout the zoom range, you'll need to go with a different lens (for example, Sigma makes an 18-55mm f/2.8 DC lens).
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Old Jun 10, 2009, 6:23 AM   #4
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Cheers for your replies JimC. I now know to only look for HSM on Sigma and BIM for Tamron as the AF-S type Nikon lenses. I'll be mostly shooting my two kids so autofocus is vital for me. And thanks for clearing up the question of the Sigma DC lenses.

I wondered about the 18-50mm f/2.8 from Sigma but wondered if it was really worth getting a second lens that covered the same focal range as the kit lens (18-55mm AF-S non-VR that comes with the D40). On another forum, the recommendation was to get a flash, but TBH it would be nice not to have to fool around with another piece of equipment. At least for the time being.

Then of course there's the 35mm or 50mm prime question as well.

I haven't even got my hands on the D40, and there's already 5 or 6 other lenses that I HAVE to buy !!!

Matt
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Old Jun 10, 2009, 6:30 AM   #5
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You've got the basic idea correct.

And this is the whole point of why the manufacturers bother to quote 35mm equivalence, because sensor and film size varies so much that it's useful to have a reference point. And as 35mm film was the most common film format, it kinda made sense to use it as the reference point when talking about focal length equivalence for digital cameras.

So yes, to convert to 35mm equivalent you must multiple the lens' actual focal length e.g 18mm x1.5 (the DX crop factor) to get a 35mm equivalent of 35mm.

And of course with a D700/D3 camera which is FX format the crop factor is 1 (no crop) so you would use a lens of 28mm focal length to get that same angle of view.

As I'm sure many people pointed out on DPR, the equivalence only extends to the field of view covered by the varying focal lengths. All of the other characteristics of the lens are not directly equivalent.

I would strongly recommend that you stick with the 18-55 kit at first. After a few months' of experimentation you will certainly get a good feel for the limitations of the aperture and focal length range and be in a better position to decide what you need. Similarly with an external flash; you may need one but you may find that the built-in flash is sufficient for your purposes and prefer to spend your money on something else. For there will always be a long list of things that you could find some use for. :-)

Last edited by peripatetic; Jun 10, 2009 at 6:33 AM.
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Old Jun 10, 2009, 6:40 AM   #6
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Cheers peripatetic. Thats very sound advice. Of course, us noobs want to know how to ensure that our photos will turn out perfectly without having to learn or gain experience. Far easier to throw money at the 'problem' !!

Thanks again.

Matt
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Old Jun 10, 2009, 6:49 AM   #7
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You have a wise approach.

It is a very common pattern that we see on these forums that someone moves from a P&S camera to a DSLR and things go wrong.

Usually the complaint is about unsharp photos. The immediate reaction is that there is something wrong with the camera or the lens. The immediate solution is to spend more money on a sharper lens or better camera. Of course this doesn't work, because it is not the root of the problem.

When someone who is getting very good results from a P&S camera steps up to a DSLR it is to be expected that things will get worse before they get better. A DSLR is much harder to "drive"; the level of skill required from the operator is higher in order just to get the same results as they were previously getting. Once the skill is gained however it is possible to progress far beyond your previous level.

So that's my advice. Don't get bummed out if things don't look great from frame 1. Give yourself a chance to learn and experiment. By the time you've hit frame 5000 your results (even with the basic kit equipment) are going to be much better. And you will know absolutely whether you need a telephoto lens, or a macro, or a flash, or a super-wide, or a fast prime or (most likely) all of the above.

Of course it's possible you might get a duff camera or lens, but it is far, far more likely that any immediate decrease in the quality of your output will be because you are adjusting and learning.
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Old Jun 10, 2009, 9:18 PM   #8
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I wondered about the 18-50mm f/2.8 from Sigma but wondered if it was really worth getting a second lens that covered the same focal range as the kit lens (18-55mm AF-S non-VR that comes with the D40). On another forum, the recommendation was to get a flash, but TBH it would be nice not to have to fool around with another piece of equipment. At least for the time being.

F/2.8 (or F/1.4 primes) lenses are not magic. They will extend low light useability somewhat, but since you are talking about taking picture of children, who are notoriously fidgety, you will almost certainly find that the flash will be a much more useful accessory.

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Old Jun 10, 2009, 9:28 PM   #9
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Quote:
Originally Posted by VTphotog View Post
F/2.8 (or F/1.4 primes) lenses are not magic. They will extend low light useability somewhat, but since you are talking about taking picture of children, who are notoriously fidgety, you will almost certainly find that the flash will be a much more useful accessory.
I use my Tamron 17-50/2.8 for available light shots of my grandchildren all the time. The trick is to shoot continuous. You'll have a lot of shots that are wasted because of motion blur due to subject movement, but you'll also get some good ones in between. And probably more than you'd get waiting for a flash to recharge.
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Old Jun 11, 2009, 2:37 AM   #10
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Theres a webshop in France that says the Tamron 17-50mm f/2.8 is not compatible with the D40. Is this true?

I'm pretty sure that I'm going to stick with the kits lens over the summer (may try and rent something big - 300mm to take pics of the family surfers), but I'll be getting an SB-400 for sure. I have white walls and a fairly low white ceiling in my flat so snapping the kids with the D40 + the SB-400 should be a good learning experience.

Like VTphotog, I also wonder how wise it is to get a second lens that covers the same range as the kit lens. From the pictures that I have already taken, I've found that most are wide-ish (28mm) so I'd be tempted to get something even wider for a second lens.

There is also the issue of my accountant (the wife) looking at the books.
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