The first of many...starting with SLR lenses

Well, here comes the first of many questions, I can't promise you they will all be intelligent :D

After a period without photography, I've finally decided to go digital and got myself an a700 last week. The main reason for buying Sony was that I have a Minolta Dynax 800si and Dynax 7 in my cupboard along with 5-6 lenses that I by a stroke of luck heard would fit a Sony DSLR. The chap in the camera shop told me though that I would have to add 50mm to the focal length of my old lenses??? That for example my Sigma Ex 105mm would now work as a 155mm lens on a DSLR. Why do I gain this additional focal length? It would seem a benefit for most lenses apart from my wide-angle lens. The one I have is 17-35mm, but I can definitely see that that's not what it is on the a700. The a700 came with an 18-70mm lens, which certainly visually appears to cover the range of my old wide angle lens, so is the latter of any use to me now? What about aperture? Does the f-stop of a lens remain the same?

Mette

The guy at the store didn't explain it quite right. The sensor in your a700 is smaller than a 35mm frame. Because of this, we get a "crop factor" with many digital camera's. Now, the focal lengths don't actually change at all. 100mm is 100mm is 100mm. But because we are all so used to the field/angle of view with 100mm on 35mm, we use the crop factor to help us determine what we might see with the DSLR. In the case of the a700, the factor is 1.5. So take 100mm, multiply by 1.5, and you get 150. So with a 100mm lens on the a700 you are getting the equivalent view as if you had put a 150mm lens on a 35mm camera (or "Full Frame" DSLR).

Do this for all your lenses.

17-35 x 1.5 = 25.5-52.5
105 x 1.5 = 157.5
18-70 x 1.5 = 27-105

The f stops are not affected by this.

It's not adding 50mm, you have to multiply x1.5 to get the equivalent focal lenght (it's actually a "crop factor") to your 35mm camera. Your 100mm is equivalent to a 150mm in the A700
Since the sensor of the camera is smaller than what you know as a negative, you are not capturing everything.
This is why the A900 is so important, because it's a "full frame sensor", it's the same size as a negative, you don't need to use the crop factor on this camera.

Thank you! Wish I had you down in my local camera shop :) That makes a bit more sense now. The 18-70mm came with a700, so I still need to do the maths for a new Sony lens? I suppose the new field of view will become second nature soon and I won't be constantly calculating (I'm a mathematician so it's not really a problem anyway LOL). Is there anything else I should be aware of when using my old lenses?

Mette

Ah, so when I find a rich man to marry and can afford the a900, I can use my 17-35mm again to full effect!

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Originally Posted by Pink Dragonfly

The 18-70mm came with a700, so I still need to do the maths for a new Sony lens?

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Yep. Every lens, whether or not it was made for cropped sensor camera's.

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Originally Posted by Pink Dragonfly

I suppose the new field of view will become second nature soon and I won't be constantly calculating (I'm a mathematician so it's not really a problem anyway LOL)

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Yep. I don't even think about it anymore.

Quote:

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Originally Posted by Pink Dragonfly

Is there anything else I should be aware of when using my old lenses?

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Be careful about Chromatic Aberration with subjects against a bright background. This is a weakness of some older glass. If you use filters on your lenses make sure they are high quality multi-coated filters. We got away with cheap filters with film, but with digital you need the good stuff or you will find ghosting, flaring, and other image anomalies.

Quote:

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Originally Posted by Pink Dragonfly

Ah, so when I find a rich man to marry and can afford the a900, I can use my 17-35mm again to full effect!

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Indeed! Although unless you are seriously considering going to a full frame DSLR I think you'll find happiness with the a700.