D200 w/18-200 - first macro attempt

Started May 9, 2006 | Discussions thread
Gabby498 Forum Member • Posts: 75
General Rules of Thumb

Still waiting to receive my D200 so I cannot give you personal experience with your setup. Your EXIF indicates that this image is taken at f/8 at 1/200th .

Since you said that this is your first attempt at macro I will mention a few things you need to consider when working close to your subject. These are some general rules that can contribute to the sharpness/softness that are not specific to a lens or a camera.

(1) To achieve maximum depth of field, DOF , your point of focus should be about one third of the way into the area that you want sharp, not at the front or at the back. I have never measured an Iris bloom but assuming six inches of depth your point of focus should be at about two inches in from the closest part of the blossom. You can get real hung up on hyperfocal distances and charts, but this is a general rule of thumb—about one third into the depth that you want to be sharp.

(2) DOF or area of acceptable sharpness is governed by the focal length of the lens. The longer the focal length the less DOF you will have. If your blossom is ten inches from your lens the 200mm setting on your zoom is going to give you less DOF than the 100mm setting at that same distance. Someone is going to rebut that because they are going to overlook the stated distance between the camera and the blossom. The size of the image on your sensor is what actually governs DOF. If you move your camera in and out from the blossom as you change focal length, as long as the blossom is the exact same size on your sensor then the DOF is going to be identical regardless of what focal length you are using on the lens. However, in general application we think of changing the focal length as changing the size of the image on the sensor. Therefore the general rule of thumb is that longer focal length settings will produce less DOF than shorter focal length settings from the same camera/subject distance.

(3) All lenses have an optimum aperture usually in the area of one to two stops from your maximum aperture where the lens is the sharpest. On your 18-200 it is probably around f/8 or f/11 (above someone suggests it is below f/13). As you stop the lens down from there DOF will be increased but the all over crispness of the lens will slightly degrade. At your smallest f-stops the decrease will be exasperated due to refraction, light bending around the aperture opening.. Your EXIF indicates that refraction is probably not applicable here.

(4) You have very soft light on the blossom. Lighting that has more contrast will contribute to more apparent sharpness or your ability to see sharpness in the image. You also do not mention what type of focusing you are using. Under some conditions a subject lacking contrast will be difficult to focus on as accurately

(5) You may also have a little motion blur. A long stemmed plant like the iris is easily affected if there is the slightest air movement. Any motion is considerably magnified by the closeness of the subject to the camera. At ten inches from your subject, the movement of only one three hundredth of an inch produces the same amount of blur as movement of thirty second of an inch in a subject one hundred inches from the camera. I mentioned that because the three very delicate petals in the center of the Iris would be highly susceptible to air movement and all three appear to be soft all the way to the base of the petals. If it were DOF alone you would think that they would get somewhat sharper at the base. I am assuming this was shot on a stable tripod although you do not say for sure. Camera movement follows the same rules.

I think I would find a subject with more contrast and more stability before I got too concerned about the apparent lens sharpness of this particular image. Just a thought. My solution in the field would be to back off slightly, get the DOF to cover, and then crop back in during post processing.

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