The Big Surprise
A couple of weeks ago I picked up a new objective on eBay that had all the right specs but I was really doubtful about the image quality, just how much sharpness and correction can a buyer expect for $17? I felt lucky so I ordered the lens anyway and took the risk. Once I had a chance to make a few images with the lens, as you can see below, it turns out that $17 buys a heck of a lot of image quality in 2017.
Why a microscope Objective?
For magnifications of 4x and higher I prefer to use to microscope objectives. They are a great cost effective way to move into high magnification macro photography. They are designed to cover a certain magnification range and a small field at the highest resolution where normal lenses must cover a bigger range and larger field but at a lower resolution. The downside to using objectives is that they have a very shallow DOF because of their large apertures and lack of an aperture so they are best used for flat subjects or for 3D subjects you would want to use image stacking best results.
Amazingly this objective produces clean sharp details across the frame. Chromatic aberrations, purple and green fringing, are almost unnoticeable and very well controlled. The CAs or chromatic aberrations are better corrected than some expensive objectives that I have tried and easily removed using post processing software. Flare is very well controlled thanks to a deep barrel and recessed front element. Corner performance is decent. Field curvature was flat as it should be for a Plan objective. Excellent results overall.
FYI most finite type microscope objectives are designed to be used with eyepieces that correct for chromatic aberrations. Nikon CF, CFI and a few Olympus finite objectives are corrected for CAs within the objective so they make great lenses for macro photography.
The sensor coverage with this lens was another surprise! This objective has a large enough image circle to cover a full frame sensor, see the pencil tip image above was made with my Nikon D810 when most other 4x objectives I own, even some of the most expensive objectives, barely cover a APS-C size crop sensor and even worst yet many simply don't give sharp results beyond an even smaller image circle!
All objectives and lenses produce a circular image field and the diameter has to be larger than the diagonal of the sensor otherwise it will vignette ( the corners of the image will be dark). For a full frame sensor the lens must have an image circle larger than 43 mm, and for an APS-C frame, the image circle needs to be at least 27 mm.
Finite objective like this one normally have smaller diameter image circle when compared to newer infinite type objectives. New objectives usually have a OFN or Objective Field Number (the Maximum field of view) of only 25 or 26mm, some are as small as 13mm! Don't forget an FX sensor like the D810 needs a 43mm diameter image circle.
Since this is a finite designs objective the installation is very simple, you just need 3 additional parts, all of which can be purchased on Ebay.
1. Objective with RMS mount
2. RMS adapter - RMS female threads and 42mm x 1 male threads. This adapter converts the RMS mount to a standard M42 mount.
3. Extension tube - 42mm mount. The spec calls for 160mm but this is total extension from the sensor the mount so most installations will need slightly different lengths.
4. Camera mount adapter - 42mm female mount to male camera lens mount.
This is a finite objective so mounting is very simple requiring only 160mm of total extension from the sensor and a couple of adapters. To make the images in this post I used the simplest method possible with parts I had on hand, RMS adapter, extension tube and camera mount adapter. Thats it.
This is a biological objective and was designed to be used with a coverslip but at this at this low NA there is no performance penalty in not using on so no need to worry about it when installing the objective.
There are two types of 42mm thread mounts, M42 lens mount which is 42mm x 1.0 (42 mm diameter, 0.75 mm thread pitch) and T-mount which is 42mm x 0.75. All of the three items in the installation diagram are listed as 42mm but you should know that the objective adapter is actually 42mm x 1mm better known as M42 thread mount, the extension tube and camera mount adapter are actually T-mount threads but luckily the M42 threaded adapter screws into the T-mount tube all the way in and even bottoms out so they work together perfectly even though they shouldn't technically but its not surprising due to widely varying manufacturing tolerances on made in China parts. You might see items with T-threads sometimes described as M42 x 0.75 on Ebay, which is the metric description of the T-mount threads.
This objective is really easy to use thanks to easy to adapt RMS mount and the small 18mm diameter front, my Canon MP-E 65 for comparison, measures 62mm across the front! The lens has a working distance of about 18mm tip to subject, thats a little less than average for the other 4x objectives that I own, most are about 20-30mm, but still that's enough to make lighting pretty easy. Handholding is always possible but the depth of field is so narrow at this magnification I recommend image stacking and some type of camera stand for best results.
What I Like:
Excellent value, good results without spending a lot of money
Easy to adapt RMS mount
Finte design so only the objective is needed and adapters, no tube lens is required
1 year warranty from Amscope (returns are accepted with a restock fee)
Good sensor coverage
The small nose cone makes lighting very easy
Where to Get One
This objective is available online for the last couple of weeks at least from a retailer Amscope for $17 with free shipping.
Amscope Ebay store link: http://www.ebay.com/itm/381613828736?
Amscope also has a store on their website (looks like they are in stock now): http://www.amscope.com/4x-plan-achromatic-compound-microscope-objective-lens-1.html#
My friend Stuart informed me that the Amscope site was showing this objective as sold out but they are still available here:
Amscope on Amazon with prime!: https://www.amazon.com/AmScope-Plan-Achromatic-Objective-Knurled/dp/B01G4CCGRC/
Update April: Two days after I posted this Amscope is now sold out, that was quick! Amscope imports their products from China and there are a couple of sellers on Ebay located in China that sell a similar objective but there is no way of knowing for sure if it is the same objective. There is one seller that offers a very similar objective for $15 plus $2.99 for shipping. Use the search terms: 4x Biological-Microscope-Plan-Achromatic-Objective-Lens-20-2mm. Let me know if you do buy one from a Chinese seller, I am curious to see if it is the same lens.
Update May 2017: Looks like the extremely similar looking direct from China objective is not the same objective but, good news, Amscope has the original objective back in stock as of May 19th!
AmScope PA4XK-V300 4X Plan Achromatic Objective Lens with Knurled Ring
Cost as of March 2017: $17 with free shipping.
NA or Numerical Aperture: 0.10
Nominal Aperture: f/4
Effective aperture: f/20
Tube Length: 160mm. This is total distance from the sensor so subtract the FBD (flange back distance) of 44mm for canon/sony and 46mm for Nikon.
Cover Glass Thickness: 0.17mm. There is no performance penalty for not using a coverslip with an NA this low.
Optical Working Distance: 29.04mm (the lens has a deep barrel and recessed from element so the actual working distance is less)
Real working distance: approximately 16mm from objective tip to subject or 18mm with hood removed.
Nose cone: removable. Unscrewing the hood will add 2mm more working distance.
Type: Finite. No tube lens is needed.
Color correction: Achromat
Field correction: Plan
Mount: RMS threads with DIN/JIS standards
Brand Name: AmScope
Model No: PA4XK-V300
Packaging: generic white box and plastic tube
Made in: China
What do the Designations Printed on the Objective Mean?
Plan: Flat Field correction
0.10: Numerical Aperture of the objective. The higher the NA the better light-gathering capacity of the objective, the better details of the image, the shallower the depth of field. The NA of this lens is equal to a nominal aperture of f/4.
160: This number identifies this as a finite tube length objective. It is designed to be used with a total of 160mm of extension from the sensor the objective mount.
0.17: This figure is the thickness, in millimeters, of the cover glass that the objective was designed to used with. With an NA of only 0.10 you can use it without a coverslip without any negative affect.