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Old Sep 8, 2011, 11:20 AM   #1
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Default DSLR Camera for nadir Aerial Imagery

Hello everyone,

I'm pretty new to the camera world, but I've been researching cameras for a while now and thought I would post a question to get some feedback. I'm looking for a camera (and lens) for a very specific task. I want to take nadir (straight down) pictures from around 200-400 feet above the ground, obviously outdoors. I want the sharpest, highest resolution picture possible with the largest field of view and the least amount of distortion (if any). After much research, I am leaning towards some kind of Canon DSLR and either a Rebel T2i, 60D, or a 7D. I really want the biggest sensor I can afford, but I can't afford a Full Frame sensor at this time. I don't care about all of the extra features the camera has, I just want to shoot in manual mode with a fixed focal length lens. I will only be playing with the shutter speed, f/stop, and a few other settings depending on the time of year. All of my pictures will be taken with a similar sun angle so once I get the settings right, there won't be much to change.

Any and all suggestions and/or input is appreciated. I will probably post a question in the Canon Lens forum to get better feedback on what type of lens to choose.

Thanks to all in advance!
-Steve
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Old Sep 8, 2011, 12:24 PM   #2
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Well... all 3 of those models (T2i, 60D, 7D) are using a Canon 18MP APS-C Size Sensor. So, I doubt you'd see much difference in image quality between them for still photos (since I'd assume that things like AF speed and tracking, burst mode, etc., are not really an issue for that kind of shooting).

I think I'd probably lean towards the 60D since it's the only one of those three models with an articulating LCD, which might come in handy if you want to try using Live View for some of your photography.
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Old Sep 9, 2011, 5:15 AM   #3
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Quote:
I want the sharpest, highest resolution picture possible with the largest field of view and the least amount of distortion (if any).
The largest field of view lens are fisheye, which brings distortion. Rectilinear wide angle lenses also bring distortion - especially at the edges. This trails off at around 28mm. Prime lenses will be sharper than zoom lenses in general.

Do you require auto focusing and auto aperture? Manual lenses are cheaper.

What are the lighting conditions? If ambient low light, you will need a relatively fast lens, which increases cost.

Will you be shooting perfectly vertical, or will you need to take slant range in to account? Off vertical will introduce additional range, and there by reduce pixel resolution. but enlarging the viewable image area.

Will the camera platform be stationary or moving? If moving - how fast (ground speed)? For the sharpest images, you are going to have to freeze the motion. Will there be induced vibration within the platform? If so, you may need image stabilization. Canon and Nikon uses lens based stabilization, while Pentax and Sony use body based stabilization.

Will this be remote - or with a person in the loop? Will you be able to make camera adjustments on the fly?

Are you taking single images, or do you want to stitch into strips? If you want to stitch into continuous strips, then you need to coordinate your ground speed with the field of view, with the continuous capture speed of the body.

Do you need the images to to be position tagged? If so then you need an interface to a GPS unit.

At 400 feet of altitude, what pixel resolution are you looking for? Using a 28mm lens at altitude, your coverage will be approximately 321 x 214 feet, thus pixel resolution would be about 0.75 inch. Assuming (you may be moving) a ground speed of 60mph (88 fps), then you would cover the 214 feet (worst case) in 2.4 seconds. Therefore you would need a sustained average 1 frame per second to maintain continuous coverage, or a lower ground speed. The body and SD card would then need to be able to write at a sustained rate of 1 FPS or something around 20MB/sec.

What is your budget?


Last edited by interested_observer; Sep 9, 2011 at 5:52 AM.
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Old Sep 9, 2011, 5:45 AM   #4
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I think I'd go with one of the 'Full Frame' dSLRs that uses Sony's 24MP sensor. That would be the Sony A900, A850 or the Nikon D3x. I think I'd also go with a catadioptric lens for increased sharpness, decreased chromatic aberration, and less bulk. Sony's 500mm f/8 Reflex would have been perfect, but it's been discontinued. There are still a lot of them (and the Minolta predecessor) available on the used market, however.
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Old Sep 9, 2011, 9:14 AM   #5
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Thanks all for the great feedback.

One thing I forgot to mention is that weight is an issue also. I need a somewhat "lean" setup. I am leaning more towards the Rebel T2i, because of its lighter weight. I would like to keep the lens under 200g or so, as well. I know that limits my choices a lot, but it is a requirement.
To answer all the questions from "Interested Observer":
I do not require auto focus. I am planning to get a manual lens, which will help with the weight.
The lighting conditions will be within an hour or two of noon, so quite a bit of sunlight. However, the camera will be pointed straight down at the ground.
I am planning to try and shoot as close to vertical as possible.
The camera should be pretty stationary when shooting. There will be some slight motion, but not moving at any recordable speed.
There will be vibration in the camera mount, but I have planned for vibration dampening fasteners which should help.
I will be capturing the pictures remotely. But, I can always get into position and take a test photo, then make adjustments before I take the bulk of the photos. Once the settings are tweaked to my liking, the entire photo shoot should take less than 20 minutes from start to finish so the sun lighting won't change much.
Yes, I am taking single images and then stitching them into a mosaic. I have done it many times on a much larger scale, but never tried it from a closer range.
I have all telemetry, including GPS, magnetic heading, barometric altimeter, etc. that will be tagged with a time to match up with the photo.
Your numbers are pretty close to what I came up with. For a 28mm lens, I get 317 x 212 feet and GSD of 0.74" For a 50mm lens, I get 177 x 119 feet and GSD of 0.41". Although, the ground resolution will be better, I don't know if I like the smaller ground footprint per image.
I plan to stop at each picture position so my speed will be approximately nothing, depending on wind conditions, etc.
My budget is $1,000-$2,000 for camera and lens... or lenses...


-Steve
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Old Sep 9, 2011, 9:59 AM   #6
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Morning Steve,

200g is not a lot. All of your best lens choices will exceed that. That said, your budget would just cover a full frame body, so I think that you will need to go with a cropped sensor.

That said, it gives you a wide latitude of bodies.

If you think that you will have a lot of vibration in the x and y axis, I do believe that a stabilized body would work well, however your resolution would decrease a bit from the Canon 18MP. Its a case of selecting where you want to optimize. Vibration in the z (vertical axis), neither stabilization approach will work - lens or body.

A cataodioptric lens would be ideal, however they cover the longer focal lengths and there is nothing really sufficiently wide for you.

Also a Canon body would provide you with the greatest selection of lenses because of its short registration distance (sensor to lens).

So with that, for an extremely sharp 28 mm lens, I would suggest a contax carl zeiss 28mm f2.8 distagon T*. If you are going with a Canon body, either a AE or MM type would work, each requiring a mount adapter (c/y to canon) - about $30. The AES version would be a bit less, but they run about $300 - $400. Note - the f2 version has a much higher price, and it much heavier in weight and larger in size.
The lens is extremely sharp in the center, with wonderful distortion control and sharpness at the edges and corners. It at least equals Canon's L glass. It also matches the best Pentax FA Limited lenses. Its 25 years old and out of production, and weighs in at 280g (has great build quality). Available on evil bay and craigslist.

Now, that is just about the sharpest lens you can get. There is also a 25mm f2.8 Distagon version. It will go for a bit more. The current version of the same lens sells new from Zeiss for $1,200 but its a f2 lens.
Here is the Canon EF 28mm - at about $250 and 115g
Overall, the lens is going to be a bit more important to you than the body. It collects and focuses the light, and defines the distortion you will see.


Last edited by interested_observer; Sep 9, 2011 at 10:13 AM.
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Old Sep 11, 2011, 11:30 AM   #7
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Thanks again for all the great information!
I agree, the lens is more important to me than the body.
One last question.
Since I'm going to be using a cropped sensor. Will that affect my field of view (on the ground)? For example, will the 28mm lens have a field of view like a lens 1.6x as long?

Thanks,
Steve
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Old Sep 11, 2011, 1:48 PM   #8
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Morning Steve,

Yes, using a cropped sensor with a 28mm will give the appearance (field of view) of a 44mm lens (on a full frame sensor). However the numbers I gave you for the 28mm lens took that in to affect already - the 321 x 214, (it was done off a photographer's calculator using a crop factor of 1.6).
Your only other choice is to try to find the oldest cheapest full frame Canon or Nikon body available. However, they still command a premium and I double that you will be able to find one that will fit your budget. It will also be substantially heavier in weight.

The other thing you need to keep in mind is that a 28mm lens is a 28mm lens no matter what body (or sensor size) it is mounted on. The edge distortion in wider angle lenses is in the lens, in terms of bending the light around the edges on to the sensor plane. So, no you can not really go with a wider lens with a cropped sensor (if you still want the best absolute image quality and sharpness), i.e., going with a 18mm lens so that when projected on to the sensor it provides the field of view of a 28mm lens. Wider angle lenses are expensive just for that reason. Do a search for the Contax Carl Zeiss 18mm lens. When you find one take a look at the price - its twice the 28mm.

The lens' focal length would be more critical for the full frame lens. Cropped sensors, use less of the lens' face and more of the center of the lenses' projected image circle onto the smaller sensor. So, there is a good possibility that you could get away with a shorter focal length and not really detect a difference.

Now, do I really think that you will be able to tell the difference? I don't know, however you asked for the sharpest. Personally, I would pick up a reasonable good condition Tokina for the body that you select or a OEM lens and give that a try (up to 17mm - maybe even a cheap 18-55 zoom for a test - you can always resell it - if the zoom looks good, you can always then look for a prime). I am betting that would probably be good enough, however you would need to do some testing to see if it meets your needs. Given your high noon shooting window with the sun angle, lens speed will not be critical. An f4 lens would be just fine. I am enclosing a link for the Tokina 17mm prime that comes in a variety of camera mounts (canon, nikon, pentax). Its sharpness, image quality and distortion control is wonderful.
Tokina, Canon and Nikon all have very good 28mm lenses...

Another possible route is using a cheaper lens that is very widely used, that has a lens correction profile for use in post processing. The way this would work is you take the image, and then back in the office, use a post processing utility to apply the lens correction profile, and that would adjust the image to reflect the known and measured distortions of the lens.

Photoshop. aperture and Bibble labs come to mind that support lens correction profiles. They all have free trial downloads for you to try out.

What ever you do, a cheap and quick test would be to go purchase some 10' 2x4 that are straight. Arrange them to be in the frame you will shoot. You can also do this with some yellow police tape from Home Depot - you can get longer runs of this and nail it to the ground in a VERY straight set of lines. Then shoot some images. You should be able to tell very quickly as to the distortion you are dealing with.

Bottom line, a 28mm lens would have the basic optical properties to hold the distortion at a minimum. Wider focal lengths then you are going to need to possibly do some additional work, depending on how demanding your needs are.

... hope that helps...


Last edited by interested_observer; Sep 11, 2011 at 2:05 PM.
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Old Sep 11, 2011, 4:14 PM   #9
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Since you're talking about a 28mm lens on a Canon APS-C body at a height of 200 to 400 feet above ground level, each image will cover from about 150 x 100 feet to about 300 x 200 feet, respectively.

Is that what you were planning on getting?

With a short focal length lens, getting the altitude right will be important.
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Last edited by TCav; Sep 12, 2011 at 6:41 AM.
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Old Sep 12, 2011, 6:40 AM   #10
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Also, with only 200 to 400 feet to play with, changes in the topography while you maintain your true altitude, will make images that might be tough to stitch together.
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