Zoom vs Fixed. Advice Requested

Discussion in 'Photography Forums' started by DPinBucks, Feb 5, 2009.

  1. DPinBucks

    DPinBucks
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    I’d like to get a new general-purpose lens for my EOS 400D, with a budget of about £300, but am in two minds over zoom versus fixed focal-length.

    Basically, I’m asking about digital vs optical zoom. I’ve always thought that cropping a wide picture is not as good as zooming the lens to the size you want, but is that in fact true, especially with SLR lenses, given that zooming is always an optical compromise?

    What I mean is, if I bought say a 50mm fixed lens and used Photoshop to crop to 200mm, would that give better quality than using a 50-200mm zoom?

    I know there are other issues, such as portability/weight (which doesn’t worry me much), and the fact that you can’t get wider than than the lens minimum FL (which does worry me a little).

    An alternative I’m considering is the Sigma 18-200mm stabilised lens, which does give wide angles. Does anyone know much about that?

    Thanks in advance. :hiya:
     
  2. fyonn

    fyonn
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    a 50mm fixed (prime) lens "zoomed" (cropped) to 200mm will only be using a small bit of the picture in the middle, thus most of the pixels on the sensor won't be used for the final picture. the 50mm lens will struggle to resolve sufficient detail at that level of cropping.

    a zoom lens that goes up to 200mm will be using all of the sensors pixels at 200mm to make up the image. which means there is more information available to create the image.

    the 50mm lens is likely (but not definately) to create a better quality image when used as what it is, but if you're going to do massive crops then you're not going to get the best out of it.

    I would say that a 50mm prim lens vs a 50-200 zoom lens at 50mm, the 50mm prime would win.

    a 50mm prime lens cropped to the same field of view as a 200mm lens vs a 50-200mm zoom lens at 200mm, the zoom lens would win.

    if you had the 50mm lens cropped to a 60mm field of view vs a 50-200mm lens at 60mm, then the 50mm lens might win as the lens is better quality, but if you start cropping too much, then you lose too much resolution.

    *however* prime lenses typically have bigger apertures (how much light they let in, smaller numbers let more light in). a consumer 50-200mm zoom lens is likely to have a max aperture of F4 to F5.6. a professional 70-200mm zoom will have a max apertue of F2.8.

    a 50mm prime lens is likely to be F1.8, it could easily be F1.4 or even F1.2. at F1.4 it wuold let 4 times as much light in as the pro F2.8 zoom, and 3-4 times as much light in as the F4-F5.6 consumer zoom.

    this means you can work in much lower light, have much faster shutter speeds, get much shorter depth of field etc.

    it's horses for courses though. different lenses have different uses. a 50mm F1.4 would be a great choice for portraits or low light street photography, but probably not much cop for catching a picture of a sparrow on a branch 100 yards away. the 50-200mm zoom would pick off that sparrow, but you'd need a tripod to get anything out of it at dusk.

    but then, the reason you own an SLR is so that you can change the lenses, depending on what you want to do :)

    don't know how much that helps?

    dave
     
    Last edited: Feb 5, 2009
  3. fyonn

    fyonn
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    oh, and incidentally, I've got the nikon 18-200mm stablised lens on my nikon and it's a great general purpose lens which I like a lot. I'd still intend to pick up a 50mm F1.8 at some point though :)
     
  4. anthony566

    anthony566
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    I use an 18-250mm 90% of the time at the moment and its first class...
     
  5. DPinBucks

    DPinBucks
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    fyonn: Many thanks for a couple of very comprehensive answers. :thumbsup:

    Zoom it is, then. I'll have a look at the Sigma.

    I used to have a couple of lenses on my old EOS film camera, but tbh in practice couldn't be bothered to keep swapping them around, which is why I want a general purpose all-day lens.
     
  6. Alistair

    Alistair
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    Optical zoom is always better than digital zoom, period.
    To correct your terminology slightly, digital zoom is more often to do with compact type digital cameras where you have an optical zoom plus a digital zoom feature. All digital zoom does is crop the image within the camera to make it appear like you have a longer focal length. With a digital SLR, cropping is the correct term as there is no 'digital zoom' option in the camera. Cropping is always done once the image has been downloaded to your computer.

    Cropping is always a compromise, whether you have a 10mp 400D or a 21mp 1Ds Mark 3. The more you crop the image, the more you reduce the quality of the image and reduce the resolution of the image. Images shot at high ISO also do not suffer cropping well either.

    With a good quality, modern, zoom lens, there is very little optical compromise. However, zoom lenses fall into 2 categories, variable aperture and fixed aperture. For example, with a Canon 100-400mm zoom lens as you zoom out from 100mm to 400mm the maximum aperture available drops from f4.5 to f5.6 but with a Canon 70-200/2.8, f2.8 is available through the zoom range.

    If you did this, you would reduce the image resolution from say 3888 pixels wide to 972 pixels wide. If you used the 200mm zoom lens you would have a full 3888 pixel wide image which much more detail. Ergo, having the right focal length for the shot is critical to a quality, detailed image. You can get away with significant cropping if the image will only ever be used say for web display at 700 pixels wide or very small prints.

    Not sure I fully understand this, if you buy a zoom lens then by virtue of the lens you have a minimum focal length and a max focal length. If you want say wider or longer, then you need a new lens.

    The Sigma 18-200 is what is known as a super zoom. It does wide angle, ie 18mm and medium telephoto, 200mm, all in one. However, with super zooms there are always compromises in lens performance.
    Personally I prefer shorter range zooms, say 17-40mm for a wide angle zoom, 70-200mm for a mid range zoom as these often give fixed apertures which is important to me as I do shoot professionally.

    You also have to take into account the sensor size on your camera too. Using the example of the Sigma you mention, on a full frame digital SLR 18mm would indeed be 18mm which is very wide. However your 400D has a cropped sensor, what is known as a 1.6x crop, therefore 18mm actually equates to 28.8mm which is still wide.

    Hope this helps a bit.
     
  7. fyonn

    fyonn
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    thanks :)

    as alastair says, the superzooms do have some compromises, but for a general photographer using consumer level equipment, I think they are a good choice.

    dave
     
  8. Calzor Suzay

    Calzor Suzay
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    Wouldn't the cropping vs Zooming give you a totally different photo?

    A photo with a 50mm cropped to the bit you want at approx 200mm will look fairly flat in comparison to a photo taken at 200mm.
     
  9. Alistair

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    Yes, plus depth of field within the image will change as well due to the different focal length used.
     
  10. johndow

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    Someone has already staed that the 50mm prime will have advantages over the 50-200 range zoom in that at max aperture it will be much faster and let in up to 4x the light of the zoom.

    However, if you take a shot wide open, you are really limiting your depth of field. Should you decide that you want to crop this photo, unless the focal point of the cropped image is in the same plane or the same point as that of the original, the chances are you will have an out of focus image. Therefore the 50mm prime is not really advantagous at this level of cropping. If you want to ensure you get a sharp image by cropping a 50mm shot to 200mm, then you would need to choose a small aperture which limits the light and leaves you with a flat shot (as stated by spod).

    If you chose to use a zoom at 200mm, you could use a aperture of say 5.6 and get your focal point sharp, and have a picture with some depth to it (blur the background etc).

    So, at say 200mm, in terms of sharpness (majority of the time) and DOF/ degree of background blur or isolation, i would choose my 55-250mm zoom lens everytime.


    Only my opinion.

    JD
     
  11. Yandros

    Yandros
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    Wrong on both counts I'm afraid. Perspective and depth of field have absolutely nothing to do with the lens used. The ONLY factor that matters is the distance from the camera to the subject.

    If you shoot the same view with a 50mm and 300mm lens, and crop the 50mm, the result will be the same. The compression effect of longer lenses is due to the fact that you are able to stand further away from the subject, not anything magical about the lens.

    As for the DOF issue - check out this article...

    DOF2
     
  12. Yandros

    Yandros
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    An 18-200 isn't a bad idea actually. I would point out though that you might not need a very long zoom. If you're looking to replace the 400D kit lens, there might be better way of spending £300 than a 200mm zoom if you don't shoot sports, wildlife etc. If you DO shoot that sort of thing, you'd be better off going longer still. I find that my 18-200 gets used in the 18-70 range, and then 200mm, and very little in between.
     
  13. sdb123

    sdb123
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    The range in between could be useful for portrait shots. :)
     
  14. ryart

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    True to a point but depth of field is do with circles of confusion. By only using part of the image the circles of confusion are effectively enlarged when the image is resized to give the same same final size as if the whole frame had been used.

    Depth of field is often, mistakenly, used to refer to the distance that is in focus. In fact, a lens only focuses at one point (point where the circles of confusion are at their smallest) and becomes progressively less sharp either side of the distance focussed on. Depth of field refers to the distance that is acceptably sharp (or unsharp).

    As for the foreshortening effect of a telephoto lens; this is do with the angle of light between then lens and the sensor. Using part of the sensor with a 50mm lens will subtend a narrower angle and give the same foreshortening "effect" as using the whole frame with a telephoto. Hence a standard lens for a 35 mm camera is nominally 50mm but for a 5x4 inch sheet film camera is 150 mm. They will both give the same optical "effect" despite the different focal length due to the film size being different.

    I hope some of this makes sense :rolleyes:, it's a long time since I was in the classroom studying photography :boring: ;).

    Edit: In the 20 minutes I spent composing the above Yandros beat me to it! I really must think faster. I still feel it is best to think in terms of the angle of view.
     
    Last edited: Feb 5, 2009
  15. Yandros

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    Yes, actually that's true. I use 105mm a lot, but in the form of a 105mm f2.8 macro. My 'dead zone' is 105-199mm. I'd be tempted to suggest an upgrade to something like a 17-85mm IS maybe?
     
  16. denno75uk

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    If you don't want to be bothered swapping lenses, but want versatility, I'd say a 18-200 or whatever would be fine for you. Plenty of people get great results with them, a friend of mine included. He used one for years and his photos were often superb. You'll find ways of working around the shortcomings and learn to play to its strengths. IMHO;)
     
  17. pet2000

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    I don't think this correct. If the distance from camera to subject is the same, a photo taken with a 50mm lens will have a different 'look' to a photo taken with a 200mm (the photo taken with the 200mm lens will look 'flat' compared to the crop from the 50mm lens).
     
  18. Yandros

    Yandros
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    It's one of the occasions where common sense lets us down - making it a very commonly held misconception.

    You're just used to being a certain distance away from subjects with a 50mm lens, and a different distance with a 200mm lens. It's rare that you opt to stand too away with a 50mm and crop heavily for the hell of it. If you did though you'd get the compression effect.

    Telephoto lens - Wikipedia, the free encyclopedia
    http://scubageek.com/articles/compression.pdf
     
  19. DPinBucks

    DPinBucks
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    Just to say thanks to everyone for their answers. :thumbsup:

    All the advice points me at the superzoom, which is what I'll go for.

    Don't let that stop the ongoing discussion. It's fascinating. :lesson:
     
  20. T0MAT01

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    A superzoom isn't always the best way forwards. From the photos I've seen, they often lack contrast and colour and usually suffer quite badly from aberations.

    In general the more zoom a lens is capable of, the lower the quality of it's images.
    So if you take a prime lens to equate to 1x zoom (i.e. none!) then the quality will drop off the further you go... so a 10x or 11x zoom (18-200mm) is quite a range for a lens to cover.
    As a guide, the standard 70-200mm zoom is less than 3x zoom which is part of the reason why they are generally of high quality.

    What I'm trying to say is that 2 lenses are better than 1.... just a bit less practical.
     
  21. pet2000

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    Hi DPinBucks, I am from a time where zoom lenses were rarely used in photography and where they were generally looked down on by 'serious' photographer (in the same way that today a 'serious' photographer looks down on anything which is not a DSLR and especially on Live View and anyone composing on a LCD screen rather than using a view finder).

    Three prime lenses will cover everything you need for general photos: a standard lens, a medium telephoto and a wide angle. On a 35mm camera this would be a 50mm, 135mm and 28mm or 24mm (the equivalent on a DSLR with APS format would be roughly 28mm, 85mm and 20mm or 18mm). I personally only ever used (and still use) a 28mm lens and a 100mm lens (I use a 20mm and 85mm a DSLR in APS format, which most are).

    If you do a lot of specialised photography, e.g. sport or stage photography, you would need some special lenses for that. I would again recommend prime lenses because they are much faster than zoom lenses and allow you to use less flash and higher shutter speeds.

    This is my personal recommendation based on the way I work. Before you spend large amounts of money on lenses, try to work with your lens which came with your camera for a while and find out this way which other lenses you need. It really depends on what type of photography you are going to do.
     

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