In this blog post we shall discuss about HOW to select the best lens for dental photography
BEFORE WE BEGIN WITH LENSES IT IS IMPORTANT TO NOTE THE BEST CAMERA FOR DENTAL PHOTOGRAPHY
In our earlier blog post we had already discussed about the kinds of cameras available in the market and which is the ideal camera for dental photography.
We reached a unanimous conclusion that DSLR cameras are the ideal choice and that buying an entry level (start-up DSLR) gives us satisfactory results with ease of operation rather than going for a high end DSLR which is very bulky and expensive.
MOVING ON TO LENSES
An ideal lens for dental photography would be the one which satisfies the following conditions:
OTHER THINGS TO LOOK FOR IN AN IDEAL LENS:
Internally Focusing Lens,
Minimum Aberrations (Spherical and Chromatic) and diffraction errors,
Lens diameter should be ideal,
Third party lenses? Good choice or bad?
Probably the first thing we look at while buying lenses is how cost effective it is but to be honest you get what you pay for.
This is because premium lenses are made up of premium materials and the glass quality is extremely good and refined.
When glass quality is good we can expect maximum quality and minimum aberration and errors from the lens and its glass elements.
CONCLUSION: Buy the best possible lens for your camera because ultimately the camera will be capturing what the lens is seeing. If the lens is not seeing properly don’t expect the camera to give you great results.
Have you ever wondered why your nose looks bigger in some images that you took with your mobile phone camera?
Well the answer to that is Phone cams are made up of wide angle lenses which give a high degree of distortion when the subject is kept very close to them.
No wonder the nose which is closest to the cameras lens looks bigger and the eyes and ears appear smaller.
This is also called “FISH EYE EFFECT”
This effect is more pronounced in ultra wide angle and wide angle lenses hence they are best avoided!
So how to avoid fish eye effect and barrel distortion?
Simple switch to the telephoto lenses.
Lenses can be placed on a focal length line as follows
It important to note that telephoto lenses can be available in huge focal lengths like 1000 mm and more.
It is however impractical to use a lens which has a focal length more than 180 mm for dental photography because more the focal length of a lens greater is the “Minimum Focusing Distance “of the lens which means that we have to stand quiet far away from the subject to shoot. In fact using a 180 mm lens for dental photography is quiet difficult because it is a very bulky lens.
The problem continues even in the manufacturing process of the lens.
Since it is understood that while shooting macro we need a very short minimum focusing distance; macro lenses face a lot of difficulty in manufacturing because they are special lenses with a shorter focusing distance compared to normal lenses. A huge challenge in arranging the glass elements.
An ideal lens for dental photography would therefore be between the focal lengths of 50 mm – 180 mm.
The lenses available in this range are:
65 mm macro
85 mm macro
90 mm macro
100 mm macro
105 mm macro
Out of all of these 65 mm macro gives a noticeable distortion when used carelessly.
It is therefore wise to stick to a focal length of 85 mm – 105 mm.
Personally I feel 100 m is the “SWEET SPOT” between achieving minimum distortion and usability in dentistry.
(A thing that might interest people who are already deeper into photography is DoF or depth of field,
It is important to note that an 85 mm lens will give give greater DoF as compared to a 105 mm lens … this might be an advantage to dentists who require full arch images on a routine basis as compared to single tooth images.
CONCLUSION: A 100 mm lens is a good lens for dental photography.
MINIMUM FOCUSING DISTANCE
The minimum focus distance is the shortest distance at which a lens can focus. In the case of digital SLR cameras, the distance to the subject is measured from the focal plane mark on the camera body, not from the front of the lens.
If you try to go closer than this distance the lens will NOT FOCUS.
VERY IMPORTANT PROPERTY FOR A MACRO LENS
(Has to have LOW focus distance)
Conclusion: We need a lens which has a short focusing distance as compared to a normal 100 mm lens … this is possible only by a 100 mm MACRO LENS
(NOTE: It makes more sense to have shorter focusing distance even for our flash systems because shorter the distance more effective is our flash)
TRUE MAGNIFICATION VALUES
Probably the most important use of a macro lens in dental photography is that it provides us with magnification values.
These values can be set even before we start making images of our cases and are extremely important in maintaining standardization in dental photography, especially before and after comparisons where we do not want that before image looks radically different as compared to after image with respect to composition.
Above: A 100 mm macro lens set at 1:1 magnification
CONCLUSION: ONLY and ONLY a macro lens can give you magnification values. We have no other choice for dental photography.
1. It is important that the lens is internally focusing and not externally focusing because in externally focusing the lens will protrude outside when you rotate the focusing ring. This causes the external flash (like ring or twin flash system) to move along with it which is not a recommended situation. In internally focusing lenses the lens remains static from outside even during the rotation of the focusing ring (auto / manual focusing) and hence the attached flash system like the ring or the dual point flash also remains static.
Externally focusing lenses look like the image on top. The dimensions of theses lenses change as we change the magnification / focusing and hence a flash attached to the lens also moves inward or outward along with the lens which causes a lot of discomfort to the dentist and the patient also feels scared
2. Usually premium lenses are the lenses that have extremely good materials in the glass elements and hence they cause minimum distortion/ aberration / errors. It is always wise to go for the best available lens for your camera.
3. It is always wise to go for a lens which has a suitable diameter for ones flash system (external macro like ring or dual point flash) rather than be worried about premium or non-premium because one cannot compromise on using an adapter which is extremely easy to loose.
For example if the (external macro) flash fits on a lens of diameter 58 mm and one has a lens of 67 mm diameter then one MUST use a step down adapter if they would like to use flash. It is impossible to use a flash on this lens otherwise. In other words without the adapter it is impossible to use the flash system on the lens of a wrong diameter.
Now the flash systems have a FIXED diameter to be mounted on.
Lenses with different diameters are available.
Hence always select the lens based on the flash dimensions and not vice versa.
(The above flash system clearly states that it is meant for a lens having a diameter of 58 mm. It is always best to have a lens of the same diameter to avoid disappointment. In case of canon there are 2 100 mm macro lenses.
One is the premium L series lens with a diameter of 67 mm
One without IS which is of a diameter of 58 mm
It is best to stick to 58 mm diameter lens so that we do not have to purchase a separate step down adapter for the flash because using a step down adapter is highly inconvenient.)
4. Lastly NEVER invest on a third party lens (Lens make of some other company rather than the same company as my camera) This is because when one shifts from the camera company to other company many features of the lens and the camera are not available for use, meaning that we are not using the lens or the camera to its full capacity!