Baffled by the Crop Factor, trying to choose wide angle zoom for new 50D, please help

[dancingmatt]

Hi all

I have read and read, and read, about the crop factor/focal length multiplier and I can't work something out so please help!

I have upgraded from a 350D with 18-55mm kit lens to a 50D and am looking to spend £500-700 on a great walkabout lens for it. It'll live on my camera and I'll be using it for a wide range of things, mainly outdoors landscapes and cityscapes I guess but also those times indoors when something begs to be shot! I also have a Sigma 105mm macro, and eventually I'll replace my Sigma APO DG 70-300mm macro with a better telezoom too.

I have been considering the EF-S 15-85 3.5/5.6 IS USM, the EF-S 17-85 4/5.6 IS USM, the EF 24-105 F4L IS USM and the EF 24-70 too.

I know you have to multiply the focal length by a crop factor of 1.6 to get equivalent to 35mm, all the references to crop factor refer to "what you would be used to on a 35mm camera..." - but I've never had one, I only got into photography with my 350D. So, can someone please explain simply , if I put an EF 24-xx lens on my 50D, will the wide angle shots at 24mm have the same field of view as I am used to from my EF-S kit lens at 24mm or at approx. 36mm?

Basically if the 24mm of an EF lens is a view like using my old EF-S kit lens at 36mm then it's too narrow and I'll have to buy an EF-S.

I'm off to a friends wedding at the weekend and would really like to make this decision before I go!

Thank you very much
DM

[YorkieBen]

The focal length is related to the lens. A 24mm lens wHether an EF or EF-S lens will giv the same field of view on your 50d.

The crop factor relates to your camera's sensor size.

[dancingmatt]

Thanks YorkieBen but I'm still not clear.

I know the focal length is for the lens and crop factor for the sensor. Do you mean they will give the same field of view and that the final image will be the same too, or that the image from the EF lens will be cropped and the EF-S not?

Surely an EF-S lens is designed for the 50D sensor, but an EF lens images a bigger image so it gets cropped?

[BUFF]

Surely an EF-S lens is designed for the 50D sensor, but an EF lens images a bigger image so it gets cropped?

some of the image circle of an FF lens will fall outside the APS-C sensor area & hence not be recorded but a 24mm focal length lens is a 24mm lens on either sensor size - it's just the fov that changes.

[CAT-THE-FIFTH]

Hi all

I have read and read, and read, about the crop factor/focal length multiplier and I can't work something out so please help!

I have upgraded from a 350D with 18-55mm kit lens to a 50D and am looking to spend £500-700 on a great walkabout lens for it. It'll live on my camera and I'll be using it for a wide range of things, mainly outdoors landscapes and cityscapes I guess but also those times indoors when something begs to be shot! I also have a Sigma 105mm macro, and eventually I'll replace my Sigma APO DG 70-300mm macro with a better telezoom too.

I have been considering the EF-S 15-85 3.5/5.6 IS USM, the EF-S 17-85 4/5.6 IS USM, the EF 24-105 F4L IS USM and the EF 24-70 too.

I know you have to multiply the focal length by a crop factor of 1.6 to get equivalent to 35mm, all the references to crop factor refer to "what you would be used to on a 35mm camera..." - but I've never had one, I only got into photography with my 350D. So, can someone please explain simply , if I put an EF 24-xx lens on my 50D, will the wide angle shots at 24mm have the same field of view as I am used to from my EF-S kit lens at 24mm or at approx. 36mm?

Basically if the 24mm of an EF lens is a view like using my old EF-S kit lens at 36mm then it's too narrow and I'll have to buy an EF-S.

I'm off to a friends wedding at the weekend and would really like to make this decision before I go!

Thank you very much
DM

Basically a 24MM lens on a 35MM SLR will give a similar field of view to a 38MM lens(in 35MM terms) on your dSLR.

The 15-85MM will give a field of view equivalent on the wide end to a 24MM lens on a 35MM SLR.

I would say the 15-85MM looks a good choice as you can use the wide end for group shots outside and it would be useful for interior shots in tight spaces.

[Bobster]

whatever the numbers say on your kit lens will be the same on any lens you put on the same camera...

[Blastuk]

Pretty much what Bobster said.

All lenses are specified in 35mm terms.

You only multiply by 1.6 when you stick lenses on a crop body (like your 50D)

You read it as it is on a full frame body.

[BUFF]

All lenses are specified in 35mm terms.

no, they are specified by what they physically are - focal length is what it is & is unaffected by sensor size.
The field of view varies.

[Saracen]

Surely an EF-S lens is designed for the 50D sensor, but an EF lens images a bigger image so it gets cropped?

Yes ..... but the two lenses will give the same field of view on a crop-sensor camera.

If you were to put an EF-S lens on a full frame camera, or 35mm film, the image created by the lens wouldn't cover the full frame of the sensor or film so the outer bits of the image will be dark ... vignetted..

If you put a 50mm EF lens on a full-frame or 35mm body, you get a (horizontal) field of view of 40 degrees.

But if you put that same lens on a crop-sensor body (or at least, on a 1.6 crop, because there are other sizes), then the outer part of the image the lens can create is lost because it falls outside the size of the sensor, and image you get left with gives the same field of view a (roughly) 80mm lens would give on a full frame sensor, which is roughly 25 degrees.



So ....

Example 1)

Put a 50mm EF or 50mm EF-S lens on a crop sensor body and you get a photo with a field of view of roughly 25 degrees, and it being EF or EF-S makes no difference to that.

But if you wanted a photo with a 25 degree field of view on a full frame (or 35mm) body, you'd need an 80mm lens to get it.


Example 2)

Put a 50mm lens on a full frame body, and you get a (horizontal) field of view of 40 degrees.

To get that same field of view on a 1.6 crop body, you need a (roughly) 30mm lens.

if I put an EF 24-xx lens on my 50D, will the wide angle shots at 24mm have the same field of view as I am used to from my EF-S kit lens at 24mm or at approx. 36mm?

Basically if the 24mm of an EF lens is a view like using my old EF-S kit lens at 36mm then it's too narrow and I'll have to buy an EF-S.

On a 50D, a 24mm EF lens and a 24mm EF-S lens will give the same field of view.

EF and EF-S give the same field of view on a given body. It only changes if you change body to one with a different sensor size.



The comparison with 35mm is obviously confusing for you, if you haven't come from that background, so you'd be best to ignore it unless you plan on upgrading to a full-frame body.

If you're happy with the field of view on a 1.6 body from a 24mm EF, you'll get the same from EF-S.

But you're right, the EF-S is designed for crop sensor, and either won't fit a full-frame body, or (with some 3rd party lenses) will physically fit, but will almost certainly cause some degree of vignetting. So .... if you buy EF-S lenses now, they'll be useless to you if you change to a full-frame body, and you'll wind up selling and replacing them.

Personally, I can't see that I'm ever likely to want to go full frame, but if you do want to, think now about whether EF-S is false economy or not.

[stavroshamster]

But you're right, the EF-S is designed for crop sensor, and either won't fit a full-frame body, or (with some 3rd party lenses) will physically fit, but will almost certainly cause some degree of vignetting. So .... if you buy EF-S lenses now, they'll be useless to you if you change to a full-frame body, and you'll wind up selling and replacing them.

What's the actual advantage in buying an EF-S frame then - given that they both produce the same image?

[kalniel]

What's the actual advantage in buying an EF-S frame then - given that they both produce the same image?

There is a size and weight benefit - as they can be designed to concentrate the light only on the smaller sensor rather than on the larger one.

Sometimes there is a glass quality advantage as well - because they're using smaller pieces it's easier to get higher quality.. that and because the pixel density is higher flaws in glass are more easily shown up so something that might pass muster at EF densities might not be good enough for APS-C hence EF-S.

I don't think there's a worry about false economies though - lenses typically hold their value second hand very well, much better than bodies.

[Flibb]

The way I think about crop factor is as follows.

take an image and draw a large circle on it
draw a rectange inside the circle, have it touching the left and right sides of the circle.
draw another rectangle inside the first one, about 60% of its size.

The circle is what the lens produces, take it off the camera look through it, it produces a magnified circle taken from the real world
Rectangle number 1 is what a 35mm neg, or a full fram sensor captures.
Rectangle number 2 is what crop sensor captures

The crop sensor does not magnify the image , it just sees less of what the lens produces. This makes getting very wide angled images difficult (and costly) on a crop sensor. Take a photo on a full frame camera with a 20mm lens, to get the same field of view with a 1.6 crop factor camera would need a 12.5mm lens. The 20mm lens would produce the same image on either camera, the crop sensor is just tioo small to pick up all of it.

bit of explanation here
http://www.cambridgeincolour.com/tut...ensor-size.htm

[kalniel]

The crop sensor does not magnify the image

Correct, but if you blow both images up to the same size the crop sensor will have the centre portion of the full sensors image, but magnified (at the same focal length)

Of course, if you're looking at the images in such as way as you display one pixel for each pixel of the image, then sensor pixel density will control the magnification of the image, and as crop sensors tend to have higher pixel density they usually magnify the image compared to full frame.

[Saracen]

.....

I don't think there's a worry about false economies though - lenses typically hold their value second hand very well, much better than bodies.

Indeed. What I meant by "false economy" is that buying an EF-S now because it's cheaper, becomes false economy if you subsequently (in the near future) have to sell it and buy EF lenses because you went Full Frame.

Sure, lenses hold their value reasonably well, but you still take a hit compared to what you pay new. And if you have to sell and switch up, you'll take that hit twice instead of once. So, if going FF is a reasonable possibility, I'd stick with EF lenses now, because I would then just keep them when switching up to an FF body.

As for the advantages of EF-S, I pretty much agree with your list. Lower size and weight, and hence cost, are certainly in the mix, as is the fact that the lens element sizes being smaller means that they're easier to make up to a high quality - in general, the bigger the element gets, the more it costs, and especially, the harder is is go keep image quality high at the outer extremes.

One other slightly more subtle advantage over some EF lenses - you pretty much know than an EF-S lens is going to have the benefit of modern design technology, whereas many EF lenses are much older designs and don t have the benefit of that .... though whether that is always ad advantage is debatable.

[dancingmatt]

Well, cheers all. I thought that one being an EF-S lens meant the crop factor had to be considered differently but I get it now. I must say though, some replies confused me further

Decided that 24 was going to be too wide, so I have a new Canon EF-S 15-85mm . It's slower than I wanted for my 50D but I think I'll mostly make do with the IS when possible. Fingers crossed it's as good as the reviews say!

Thanks for all your help

DM

[IBM]