BetterPhoto Q&A
Category: New Answers

Using Extension Tubes?

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March 09, 2006

The focal length of the camera lens as engraved on the lens barrel is calculated based on the camera being focused on infinity. The f numbers engraved on the lens barrel are also dependent on this stated focal length. When you focus on nearby objects the lens is repositioned further away from the film. Anytime the lens operates outside normal limits an error is induced and the engraved f number markings on the lens barrel lose accuracy. In the normal picture taking range, the error is negligible. Once you employ devices that allow a focus closer than the camera’s normal range, the error becomes noteworthy. Most camera designs stop close focusing when the error approaches 1/3 of an f stop. However, macro lens and modern cameras with through the lens metering are immune as built-in metering and/or mechanical logic triumphs.

The two biggest pitfalls of extreme close-up photography are un-sharp images and under exposed images.
First the un-sharp:
Camera taking lenses are designed to project a 3 dimensional world onto a 2 dimensional film plane. Most subjects of close-up photography are stamps, coins and like. These are 2 dimensional objects and some camera lenses will be substandard for a flat to flat conditions. Sometimes an accessory ring can be found that will allow the camera lens to be mounted backward (front facing film). Many times the inverted lens is best when taking extreme close-ups. When so mounted, all the built-in lens automation is lost. With extreme close-ups, depth of field becomes shallow. Camera movement and vibration become amplified so the mounting system must be beefy.
Under-exposure:
The error known as BF (bellows factor) is cruel. If your camera has through-the-lens-metering, you will be just fine as the camera logic will compensate. If not, you must take a meter reading and then open up the lens to compensate for BF. You need to know the magnification which is represented by letter M. The formula is BF = (M + 1) ². For example if you are taking a life size picture, known as 1:1, the magnification is 1. Applying the formula: M + 1 = 2 and 2 squared = 4. Thus the BF = 4. You handle BF just like a filter factor. Because each f stop presents a 2x change in light level, you count on you fingers in powers of 2 thus:
2 – 4 – 8 – 16 – 32 each finger symbolizes an additional f stop opening. For a BF or filter factor of 4, open up 2 f stops – for a BF of 8 open up 3 f stops – for a BF or 6 open up 2 ½ f stops.

How to find M via the viewfinder:
35mm film has an image dimension of 24 x 36 millimeters. The SLR’s view finder also has the same dimensions. Substitute a millimeter ruler for the subject. Focus and count how many millimeter markings you can see on the long dimension in your viewfinder. See 72 marks, the M = ½ : See 36 marks, M = 1: See 18 marks, M = 2 etc.

Good luck
Alan Marcus

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March 09, 2006

The focal length of the camera lens as engraved on the lens barrel is calculated based on the camera being focused on infinity. The f numbers engraved on the lens barrel are also dependent on this stated focal length. When you focus on nearby objects the lens is repositioned further away from the film. Anytime the lens operates outside normal limits an error is induced and the engraved f number markings on the lens barrel lose accuracy. In the normal picture taking range, the error is negligible. Once you employ devices that allow a focus closer than the camera’s normal range, the error becomes noteworthy. Most camera designs stop close focusing when the error approaches 1/3 of an f stop. However, macro lens and modern cameras with through the lens metering are immune as built-in metering and/or mechanical logic triumphs.

The two biggest pitfalls of extreme close-up photography are un-sharp images and under exposed images.
First the un-sharp:
Camera taking lenses are designed to project a 3 dimensional world onto a 2 dimensional film plane. Most subjects of close-up photography are stamps, coins and like. These are 2 dimensional objects and some camera lenses will be substandard for a flat to flat conditions. Sometimes an accessory ring can be found that will allow the camera lens to be mounted backward (front facing film). Many times the inverted lens is best when taking extreme close-ups. When so mounted, all the built-in lens automation is lost. With extreme close-ups, depth of field becomes shallow. Camera movement and vibration become amplified so the mounting system must be beefy.
Under-exposure:
The error known as BF (bellows factor) is cruel. If your camera has through-the-lens-metering, you will be just fine as the camera logic will compensate. If not, you must take a meter reading and then open up the lens to compensate for BF. You need to know the magnification which is represented by letter M. The formula is BF = (M + 1) ². For example if you are taking a life size picture, known as 1:1, the magnification is 1. Applying the formula: M + 1 = 2 and 2 squared = 4. Thus the BF = 4. You handle BF just like a filter factor. Because each f stop presents a 2x change in light level, you count on you fingers in powers of 2 thus:
2 – 4 – 8 – 16 – 32 each finger symbolizes an additional f stop opening. For a BF or filter factor of 4, open up 2 f stops – for a BF of 8 open up 3 f stops – for a BF or 6 open up 2 ½ f stops.

How to find M via the viewfinder:
35mm film has an image dimension of 24 x 36 millimeters. The SLR’s view finder also has the same dimensions. Substitute a millimeter ruler for the subject. Focus and count how many millimeter markings you can see on the long dimension in your viewfinder. See 72 marks, the M = ½ : See 36 marks, M = 1: See 18 marks, M = 2 etc.

Good luck
Alan Marcus

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March 09, 2006