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04-29-2009, 04:57 AM   #1
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Lenses and megapixels

Finally this week-end I will order my new Pentax DSLR.
I made a list of the lenses I could need (very few to be honest) and I started thinking about prime lenses.
A question arises: does a prime lens just waste sensor's megapixels?
I mean: if you crop your photos you'll lose pixels... if I have a 10mpixel sensor and I keep a third of my shot, I back to a 3 mpixel image.
A zoom lens, instead, works-around cropping by adjusting the focal length... achieving the possibility to use the full sensor resolution.

I would like to start using primes cause of their (usually) superior optical quality and I also know that you act in a different way when you shot with primes... but until now (got a 10x zoom compact camera) I cropped very few images (about a dozen after more than 8400 shots)... that means I always have all the sensor's megapixel available for all my photos.

Also I read reviews that talks about cameras (that have 15/20 megapixels) which are good because of their cropping possibilities. Looks strange for me to pay for tenths of megapixel in order to crop them down...

Is there a flaw in my thinking? How often is cropping needed when using primes?

(again these are question that I have because the new Pentax should become a soon as possible my companion, expecially in "official" events... I would like to have an idea of what to expect...)

Thanks,
Jenner

04-29-2009, 06:39 AM   #2
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I use primes pretty much exclusively (I use a zoom lens perhaps once a year). I rarely crop the photo in post processing, and if I do, it is very slightly.
The trick to photography is a thing called composition, which is where you get the picture you want in the viewfinder and then push the button.
04-29-2009, 06:54 AM   #3
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QuoteOriginally posted by ntx Quote
A question arises: does a prime lens just waste sensor's megapixels?
I don't think so. Shooting with a prime causes you to think differently - you're more focused on the composition. I think they actually cause less cropping, not more. The only time that cropping comes into play is when your movement is restricted - that is, you can't move closer or farther from the subject in order to frame it properly. Otherwise, just zoom with your feet.
04-29-2009, 07:02 AM   #4
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QuoteOriginally posted by ntx Quote
if you crop your photos you'll lose pixels... if I have a 10mpixel sensor and I keep a third of my shot, I back to a 3 mpixel image.
QuoteQuote:
until now (got a 10x zoom compact camera) I cropped very few images (about a dozen after more than 8400 shots)... that means I always have all the sensor's megapixel available for all my photos.
I'll leave the prime comments to others, but let me spin this on its head just as a thought exercise: why is this important to you? What do you do (or think you might want to do in the future) with your pictures that requires them to have that many megapixels?

QuoteQuote:
Also I read reviews that talks about cameras (that have 15/20 megapixels) which are good because of their cropping possibilities. Looks strange for me to pay for tenths of megapixel in order to crop them down...
I look at this possibility as being for situations where I have the exact picture I meant to take, but then discover a detail in it that makes its own excellent picture, if I'd only thought to move closer and take it at the time. With sufficient resolution, I can just crop it from the larger photo instead. It doesn't come up often, but it's neat when it does. There are similar situations, on the web in particular, where you might do that to highlight parts of a scene, since you're posting relatively small images.

(Well, I also tend to crop when my lens doesn't have enough reach and I can't move closer, but that could be fixed by just getting another lens...)

04-29-2009, 07:57 AM   #5
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Jenner,

Primes do have superior quality over zooms.
In order to cover a zoom's range, people will work with various primes.
So, in case of a zoom, you exchange quality for conveniance.
With a zoom you need less lens changes and your camera bag does not weigh as much.

It is a matter of priority and also under what type of circumstances you expect to make photos.
With primes you will need to change lenses or work with multiple bodies with a variety of lenses mounted.
A zoom is much more versatile.

If you know you'll be doing macro or portrait shots. It's easy, work with a prime.
If you need to travel light or respond instantly on situations, take a zoom.

Lots of fun with your new Pentax DSLR!

- Bert
04-29-2009, 08:30 AM   #6
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Just because you use a prime doesn't mean you need to crop. Ideally you shoot from a location with the right focal length prime--that's why they come in many different lengths.

Certainly cropping a small slice out of a larger image is going to throw out a lot of image data--very possibly to the extent that it will negate any optical advantage a prime might have had. On the other hand if you shoot with a 50mm prime when you might have used 60mm with a zoom (a mild crop), you'll still have a lot of pixels after cropping and perhaps the speed and sharpness of the prime makes the loss of pixels worthwhile. Another thing is that we don't always know ahead of time that we might crop something later, so having more pixels can improve the quality of those crops.

If your intent is to deliberately shoot with a shorter prime with the intent that you'll just crop later, that's another question...so what did you have in mind?

Many of us own a mix of zooms and primes--zooms are more flexible and allow us to cover a wide range of focal lengths--but primes offer for a single length generally higher speed and optical quality.

Last edited by AndrewG NY; 04-30-2009 at 07:54 AM. Reason: (corrected transposed words)
04-29-2009, 08:52 AM   #7
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I regularly shoot primes for any "non-event" type shooting (events I need the flexibility of a 17-85-ish zoom) I have a 30mm f/1.4 mounted on one body and an 85mm f/1.8 mounted on a second body. This combo gets MOST shots one could hope to get. With primes, you alter your position to get the framing you want so if you are composing properly, you shouldn't need to crop. If I need wider than 30, I have a 10-20 and if I need longer than 85 I have a 70-200 and a 100-400
04-29-2009, 09:55 AM   #8
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I think there are three issues with your question. One is the question of megapixels. Back when I had a K100, I found that 6 megapixels was plenty to work with both with regard to viewing and printing. Unless you are printing huge photos, you probably need no more than 6 megapixels, so you can easily crop and still be fine.

The other issue is one of when and how you crop. In general, I do not crop a photo just to make it look as though I had taken the picture with a longer focal length. Rather, I crop in order to frame the subject better, to draw attention to the subject, etc. In point of fact, I should be able to do this with the camera prior to post processing, but if I don't, it is easy to fix after the fact.

The final thing is the question of prime versus zoom quality. I think that modern zooms have excellent quality and produce great images. The difference is really speed (primes are faster) and size (primes are smaller). What you use is primarily dependent on what you need.

04-29-2009, 09:57 AM   #9
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QuoteOriginally posted by AndrewG NY Quote
Just because you use a prime doesn't need you mean to crop.
Nicely put. I suppose if you only own one prime and try to use it for shots for which is not well suited, then you'd have to crop. But the completely misses the point of using primes.
04-29-2009, 10:03 AM   #10
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I think that most zoom use is at one end of the range or the other (even for mobility limited people like me) - except maybe for superzooms.

Therefore the zoom lens essentially replaces two lenses; you get the convenience of not having to carry/interchange lenses at the cost of walking, shutter speed & image quality (assuming you get reasonable quality lenses.)

Regarding cropping vs lens changes - cropping works if light and lenses are good enough; as crop is increased the noise per viewing pixel increases and the importance of the lens' sharpness increases.

Iowa Dave

PS importance of lens quality, pixel density, and sensor size is a matter of how far one needs to push your system exposure and crop-wise. In great light it often matters little; a sunny day shot with a simple P&S can look great! The same P&S may fail miserably from the 20th row in the theater.

Last edited by newarts; 04-29-2009 at 10:41 AM.
04-29-2009, 10:23 AM   #11
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my 50mm prime looks better cropped in than my kit zoom 70-300 did zoomed in to 300 :P

when i realised this the 300 was sold.
04-29-2009, 11:05 AM   #12
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Really? That seems going a bit far. I actually tried this with my DA70, comparing to my DA50-200 and (Quantaray-badged) Tamron 70-300. The 70 was sharp, no doubt, but when cropped on my 10MP K200D, it couldn't come close to standing up to even the 200mm on either zoom. I find it hard to imagine that even the extra resolution of the K20D would really allow a 50mm prime to match a 70-300 zoom. Do you still have test shots?
04-29-2009, 01:25 PM   #13
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Thank you for sharing you point of view. It's very useful for me.

@Quension: dropping megapixels for a photo that has to go on the web isn't a problem, but when you print it on a 45cm x 30cm paper megapixels matter (and I bet you just know that). I wouldn't print on that size every single shot I do, that for sure. BTW I remember when I organized I local (and little) photographic exposition here... all my photos were compositions of many shots (from 4 to 13) blend together only to end up with 15/16 megapixels per shot... and I do that even for non-panoramical subjects. The resulting prints were great (but I spent many hours of work for each shot).

Let's take the last week-end cycling race where I was called (again) to be the "official photographer". The organization likes to print some photo (in big formats) just to remind the event... but while I was there I noted how many focal length changes I made to shot the whole race and how I needed a brighter lens (it was raining, can't go beyond 1/120s without flooding the shot with sensor noise).
And again, about two weeks ago, the local school organized a cycling event for all the children on a closed and short course: there was few good positions where I can't stay to take shots... and I "must" get at least two shot per child (parents let me going mad! ): the better the quality, the more the satisfation (many people just want to count the folds on his son lips and never care of any technical or artistic aspect...).

So I asked myself if I would be able to do the same when I'll own the K200D... maybe with a nice and bright prime... how many shots I would miss? (I'm focused on the primes cause now I like them more than zooms)

Maybe I'm just wrong thinking to use primes on those situations... but they are precise, brighter and - as you said - fast.

@Rondec: You are right that cropping could be used for better framing or move the attention in other places but... when I zoom in or out I'm just doing this: better framing and moving attention by including/excluding details... I don't zoom cause I'm scared to make two steps ahead!

Thank you again for your opinions!

Bye
Jenner
04-29-2009, 01:27 PM   #14
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I end up carrying a mixed bag (or bags) for different purposes. A few fastest primes (f/2 or better) for lowest light: 24, 50, 58, 85. Some fast primes (f/2.8) for more general work: 16FE, 28M, 35, 90M, 135. And a mix of slower (f/3.5 or worse) primes for sharpness, and zooms for versatility: 21, 200, 10-17FE, 18-250, 100-300. Plus anything else I want to try out and play with.

Many shots get more-or-less reformatted - but am I wasting pixels? Considering that the first digicams sported fewer than 1mpx sensors, I appreciate Arizona's approach to Daylight Saving Time: they have enough sunshine that they don't need to conserve any. I've seen superb photos taken with a 3mpx Nikon. I've blown up 1mpx shots to poster size (and squeezed them down to 6x9cm) and at the right viewing distance, they're fine. Are you doing coffee-table books or large calendars? If not, don't fret.
04-30-2009, 06:20 AM   #15
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yes the fa 50 1.4 was sharper than the 70-300 sigma dg...

i suspect it may have been the start of an issue with the cameras shake reduction that resulted in a replaced module a few months later.
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