Monthly Archives: April 2009

Panoramio

by · · leave a comment

The Panoramio photo sharing service is linked to Google Earth. You can upload images to Panoramio, geotag them (pick a spot on the map where your picture was taken), and they may be selected for viewing via Google Earth. They tend to only select those photos that are primarily landscapes. If a person or animal is the central element of the photo, it is unlikely to be selected for Google Earth. The selection process takes a while, with about a monthly schedule for inclusion of new photos into Google Earth, so don’t get impatient.

I’ve uploaded some of my photos to Panoramio.

Viewed 37563 times by 6694 viewers

Taming the Fuji S2 Red Response

by · · leave a comment

At FixYa.com, a surgeon asked about using the Fuji S2 with a ringflash for surgical photos. Red is easy to oversaturate on the Fuji S2, and this leads to photos that don’t yield the distinctions between tissue color that he is looking for. Here is the advice I gave:

The Fuji S2 sensor is noted for this red sensitivity. Here’s a few different approaches to getting the shot with the S2, all of which require some experimentation.

1. Set the “Color” and “Tone” Function options to “ORG”. Underexpose the shot. Progressively change the exposure compensation downward until when checking the histogram, the red channel does not show saturation at the high end. Advantages: this is the simplest approach to getting the picture. Disadvantages: the S2 already had a fairly limited dynamic range, and this will make things worse for the parts of the photo that are not red. For the surgical setting and use of a ringflash, this may not be much of a disadvantage, since a lot of the stuff of interest will be red, and ringflash illumination generally is of lower contrast than directional lighting.

2. Set the “Color” and “Tone” Function options to “ORG”. Use a custom white balance. The idea here is to have the camera adjust the red channel sensitivity itself, and leave the blue and green channels alone. To do this, start with several sheets of white paper and a red or pink marker or highlighter. Scribble with the marker across a sheet, then use that to set a custom white balance. Take a test shot of the red stuff that has been problematic, and see whether the histogram for the red channel shows that there is no saturation at the high end. Repeat this with progressively more red or pink on each sheet used to set the custom white balance until you find the custom white balance that takes enough of the edge off the red channel response. Alternative: I just tried out making a gradient across an 8.5×11″ sheet of paper going from white to about 30% red saturation. I can set more or less red adjustment in a custom white balance just by pointing the camera at different parts of the page. This seems to work OK for me. Advantage: can allow the full dynamic range of the sensor to be used. Disadvantages: the experimentation period is likely to take a while to get the best results, and the final images are unlikely to look completely natural.

3. Set the “Color” Function option to “B/W”. Use a green or cyan filter on the lens to cut the amount that the red color channel contributes to the final image. Advantages: this is fairly simple as an approach. A similar post-processing technique can be applied to the photos that you already have, by nulling out the red channel contribution and desaturating the blue and green channels to produce a grayscale image. Disadvantages: you lose the color information entirely. Since much of what you want information about is colored red anyway, the organs are likely to appear quite dark when only taking the blue and green channel contributions to the image.

Viewed 33551 times by 6410 viewers

Balancing Electronic Flash and Ambient Light Color

by · · 4 comments

Over on FixYa.com, I answered a question posed about how to set the white balance on a camera when using electronic flash indoors. Here’s my answer:

There are three possible solutions for you:

1. Just use the Daylight (direct sun) white balance setting. Electronic flash is typically close to the direct sunlight setting for white balance. Advantages: it is simple. Disadvantages: if you are balancing electronic flash with ambient light, you are likely to have an odd mixture of lighting, with the ambient lighting tending toward red (if incandescent) or green (if flourescent).

2. Get a Wratten 85 color correction gel that you can cover your flash’s head with, and use the Tungsten white balance setting. This will bring your flash output into line with existing incandescent light sources. Advantages: almost as simple as (1), and allows you to come close to matching a common indoor lighting situation. This will produce more natural-looking environmental shots if you balance the flash and incandescent ambient light contributions to the exposure. Disadvantages: large gel filters get expensive. You may be able to obtain something close to the Wratten 85 color correction filter from a theater supply house more cheaply. Roscolux #3408 is slightly weaker and #3411 is slightly stronger than the Wratten 85. If you have to balance flash with flourescent light sources, try a Roscolux in the cyan series leading to #4360. Some experimentation is likely to be required. The place I looked online offered the Roscolux in 20×24″ sheets for about $7 per sheet, a bargain compared to the optical-grade Wratten filters.

3. Set a custom white balance. This is not too difficult, and it will definitely provide the best approximation to your desired white balance that you will obtain in the field. Advantages: this will allow you to reproduce white accurately even with mixed lighting sources. Combine with gel over flash of (2) for the very best approach to balanced flash/incandescent lighting situations. Disadvantages: requires a “white” target (can be a neutral gray photo card, for example) and some additional time before a photo session.

You can obtain some striking effects by purposely mixing color temperature light sources. But day in and day out, getting a balanced shot is a skill that it pays to cultivate.

Viewed 38613 times by 7670 viewers

Thoughts on Sensor Resolution

by · · leave a comment

Originally posted to the “Nikon SLR Forum” on the DPReview site.

Moon wrote:

> DMillier wrote:
> > Yes when the frame enlarges, the pixels enlarge. But SO DOES THE
> > SIZE OF THE OPTICAL IMAGE!

> > If your one pixel on a small sensor is positioned so it covers say
> > 1 sq inch of the subject and you enlarge the pixel by say x10 what
> > happens? Absolutely nothing if you use a different lens matched to
> > the new sensor size. This is because the new optical image will
> > also be x10 larger so your one (x10 enlarged pixel) will still
> > cover 1 square inch of your (now x10 enlarged) subject...

> I follow what you are saying, but my presumption is that we were
> discussing a 35mm system. The D/1/x/h/100/30/60, 1D, etc. are based
> on 35mm systems and, as such, use the same lens system. Assuming
> the same lens is used from one sensor size to the next and the
> frames were compositionally identical, resolution would be lost.

IIRC, the formula for working out resolution with film is

1/res_total ~= 1/res_lens + 1/res_film

where resolution is given in lines/mm or some similar units.

So lets say we’ve got a lens with res_lens = 60 l/mm, and calculate numbers for CCD pixel density given Ron’s sensor sizes. For 6MP on a 15x23mm sensor, we get 1.71e5/mm, and for a 24x36mm sensor we get 6.94e3/mm. I’m going to just equate pixel spacing with lines/mm here; since this is just a before/after comparison this shouldn’t be a problem. So we have res_small = 132 l/mm and res_large = 83.3 l/mm (square root of the pixel density figures).

res_total_small ~= 1/60 + 1/132 = 41.2 l/mm

res_total_large ~= 1/60 + 1/83.3 = 34.9 l/mm

That’s a distinct drop in resolving power, yes?

But that wasn’t the whole claim made. The whole claim concerns how well an *identical composition* is resolved under the two different scenarios. And that takes us to a slightly different concept of resolution, one that is common in videography, which is the total number of lines of resolution that the system is capable of. This we can approximate by taking into account the sensor sizes and the lines/mm figures we just calculated. Again, we’re just looking at a before/after comparison, so this should also not be a problem.

total_lines = l/mm * mm

total_lines_small = 41.2 l/mm * 23mm = 948 lines

total_lines_large = 34.9 l/mm * 36mm = 1,256 lines

Hmm. It seems to me on the math that the larger sensor has the edge over the smaller sensor given the same number of pixels in both. This also explains why lenses that were adequate for use with 35mm film may not be adequate for the small sensors of the current [posted around 2003, IIRC] crop of DSLRs (D100, S2, D60). In order to get comparable performance, one needs a res_lens that is significantly higher. (To get the same total lines figure for the case above, for instance, one needs a lens capable of resolving 93 lines/mm on the smaller sensor to match the performance of a lens that resolves 60 lines/mm on the larger sensor.)

But perhaps I’ve remembered the formula wrong or messed something up in applying it. If so, I’d appreciate a correction.

Viewed 17095 times by 4470 viewers